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# This file is distributed under the same license as the FreeBSD Documentation package.
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#. type: Title =
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:1
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:14
#, no-wrap
msgid "SMPng Design Document"
msgstr ""

#. type: YAML Front Matter: title
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:1
#, no-wrap
msgid "Chapter 8. SMPng Design Document"
msgstr ""

#. type: Title ==
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:52
#, no-wrap
msgid "Introduction"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:55
msgid ""
"This document presents the current design and implementation of the SMPng "
"Architecture. First, the basic primitives and tools are introduced. Next, a "
"general architecture for the FreeBSD kernel's synchronization and execution "
"model is laid out. Then, locking strategies for specific subsystems are "
"discussed, documenting the approaches taken to introduce fine-grained "
"synchronization and parallelism for each subsystem. Finally, detailed "
"implementation notes are provided to motivate design choices, and make the "
"reader aware of important implications involving the use of specific "
"primitives."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:57
msgid ""
"This document is a work-in-progress, and will be updated to reflect on-going "
"design and implementation activities associated with the SMPng Project. Many "
"sections currently exist only in outline form, but will be fleshed out as "
"work proceeds. Updates or suggestions regarding the document may be directed "
"to the document editors."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:63
msgid ""
"The goal of SMPng is to allow concurrency in the kernel. The kernel is "
"basically one rather large and complex program. To make the kernel multi-"
"threaded we use some of the same tools used to make other programs multi-"
"threaded. These include mutexes, shared/exclusive locks, semaphores, and "
"condition variables. For the definitions of these and other SMP-related "
"terms, please see the crossref:smp[smp-glossary, Glossary] section of this "
"article."
msgstr ""

#. type: Title ==
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:65
#, no-wrap
msgid "Basic Tools and Locking Fundamentals"
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:67
#, no-wrap
msgid "Atomic Instructions and Memory Barriers"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:70
msgid ""
"There are several existing treatments of memory barriers and atomic "
"instructions, so this section will not include a lot of detail. To put it "
"simply, one can not go around reading variables without a lock if a lock is "
"used to protect writes to that variable. This becomes obvious when you "
"consider that memory barriers simply determine relative order of memory "
"operations; they do not make any guarantee about timing of memory "
"operations. That is, a memory barrier does not force the contents of a CPU's "
"local cache or store buffer to flush. Instead, the memory barrier at lock "
"release simply ensures that all writes to the protected data will be visible "
"to other CPU's or devices if the write to release the lock is visible. The "
"CPU is free to keep that data in its cache or store buffer as long as it "
"wants. However, if another CPU performs an atomic instruction on the same "
"datum, the first CPU must guarantee that the updated value is made visible "
"to the second CPU along with any other operations that memory barriers may "
"require."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:72
msgid ""
"For example, assuming a simple model where data is considered visible when "
"it is in main memory (or a global cache), when an atomic instruction is "
"triggered on one CPU, other CPU's store buffers and caches must flush any "
"writes to that same cache line along with any pending operations behind a "
"memory barrier."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:74
msgid ""
"This requires one to take special care when using an item protected by "
"atomic instructions. For example, in the sleep mutex implementation, we have "
"to use an `atomic_cmpset` rather than an `atomic_set` to turn on the "
"`MTX_CONTESTED` bit. The reason is that we read the value of `mtx_lock` into "
"a variable and then make a decision based on that read. However, the value "
"we read may be stale, or it may change while we are making our decision. "
"Thus, when the `atomic_set` executed, it may end up setting the bit on "
"another value than the one we made the decision on. Thus, we have to use an "
"`atomic_cmpset` to set the value only if the value we made the decision on "
"is up-to-date and valid."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:76
msgid ""
"Finally, atomic instructions only allow one item to be updated or read. If "
"one needs to atomically update several items, then a lock must be used "
"instead. For example, if two counters must be read and have values that are "
"consistent relative to each other, then those counters must be protected by "
"a lock rather than by separate atomic instructions."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:77
#, no-wrap
msgid "Read Locks Versus Write Locks"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:80
msgid ""
"Read locks do not need to be as strong as write locks. Both types of locks "
"need to ensure that the data they are accessing is not stale. However, only "
"write access requires exclusive access. Multiple threads can safely read a "
"value. Using different types of locks for reads and writes can be "
"implemented in a number of ways."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:82
msgid ""
"First, sx locks can be used in this manner by using an exclusive lock when "
"writing and a shared lock when reading. This method is quite straightforward."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:84
msgid ""
"A second method is a bit more obscure. You can protect a datum with multiple "
"locks. Then for reading that data you simply need to have a read lock of one "
"of the locks. However, to write to the data, you need to have a write lock "
"of all of the locks. This can make writing rather expensive but can be "
"useful when data is accessed in various ways. For example, the parent "
"process pointer is protected by both the `proctree_lock` sx lock and the per-"
"process mutex. Sometimes the proc lock is easier as we are just checking to "
"see who a parent of a process is that we already have locked. However, other "
"places such as `inferior` need to walk the tree of processes via parent "
"pointers and locking each process would be prohibitive as well as a pain to "
"guarantee that the condition you are checking remains valid for both the "
"check and the actions taken as a result of the check."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:85
#, no-wrap
msgid "Locking Conditions and Results"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:88
msgid ""
"If you need a lock to check the state of a variable so that you can take an "
"action based on the state you read, you can not just hold the lock while "
"reading the variable and then drop the lock before you act on the value you "
"read. Once you drop the lock, the variable can change rendering your "
"decision invalid. Thus, you must hold the lock both while reading the "
"variable and while performing the action as a result of the test."
msgstr ""

#. type: Title ==
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:90
#, no-wrap
msgid "General Architecture and Design"
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:92
#, no-wrap
msgid "Interrupt Handling"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:95
msgid ""
"Following the pattern of several other multi-threaded UNIX(R) kernels, "
"FreeBSD deals with interrupt handlers by giving them their own thread "
"context. Providing a context for interrupt handlers allows them to block on "
"locks. To help avoid latency, however, interrupt threads run at real-time "
"kernel priority. Thus, interrupt handlers should not execute for very long "
"to avoid starving other kernel threads. In addition, since multiple handlers "
"may share an interrupt thread, interrupt handlers should not sleep or use a "
"sleepable lock to avoid starving another interrupt handler."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:97
msgid ""
"The interrupt threads currently in FreeBSD are referred to as heavyweight "
"interrupt threads. They are called this because switching to an interrupt "
"thread involves a full context switch. In the initial implementation, the "
"kernel was not preemptive and thus interrupts that interrupted a kernel "
"thread would have to wait until the kernel thread blocked or returned to "
"userland before they would have an opportunity to run."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:99
msgid ""
"To deal with the latency problems, the kernel in FreeBSD has been made "
"preemptive. Currently, we only preempt a kernel thread when we release a "
"sleep mutex or when an interrupt comes in. However, the plan is to make the "
"FreeBSD kernel fully preemptive as described below."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:101
msgid ""
"Not all interrupt handlers execute in a thread context. Instead, some "
"handlers execute directly in primary interrupt context. These interrupt "
"handlers are currently misnamed \"fast\" interrupt handlers since the "
"`INTR_FAST` flag used in earlier versions of the kernel is used to mark "
"these handlers. The only interrupts which currently use these types of "
"interrupt handlers are clock interrupts and serial I/O device interrupts. "
"Since these handlers do not have their own context, they may not acquire "
"blocking locks and thus may only use spin mutexes."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:103
msgid ""
"Finally, there is one optional optimization that can be added in MD code "
"called lightweight context switches. Since an interrupt thread executes in a "
"kernel context, it can borrow the vmspace of any process. Thus, in a "
"lightweight context switch, the switch to the interrupt thread does not "
"switch vmspaces but borrows the vmspace of the interrupted thread. In order "
"to ensure that the vmspace of the interrupted thread does not disappear out "
"from under us, the interrupted thread is not allowed to execute until the "
"interrupt thread is no longer borrowing its vmspace. This can happen when "
"the interrupt thread either blocks or finishes. If an interrupt thread "
"blocks, then it will use its own context when it is made runnable again. "
"Thus, it can release the interrupted thread."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:105
msgid ""
"The cons of this optimization are that they are very machine specific and "
"complex and thus only worth the effort if their is a large performance "
"improvement. At this point it is probably too early to tell, and in fact, "
"will probably hurt performance as almost all interrupt handlers will "
"immediately block on Giant and require a thread fix-up when they block. "
"Also, an alternative method of interrupt handling has been proposed by Mike "
"Smith that works like so:"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:107
msgid ""
"Each interrupt handler has two parts: a predicate which runs in primary "
"interrupt context and a handler which runs in its own thread context."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:108
msgid ""
"If an interrupt handler has a predicate, then when an interrupt is "
"triggered, the predicate is run. If the predicate returns true then the "
"interrupt is assumed to be fully handled and the kernel returns from the "
"interrupt. If the predicate returns false or there is no predicate, then the "
"threaded handler is scheduled to run."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:110
msgid ""
"Fitting light weight context switches into this scheme might prove rather "
"complicated. Since we may want to change to this scheme at some point in the "
"future, it is probably best to defer work on light weight context switches "
"until we have settled on the final interrupt handling architecture and "
"determined how light weight context switches might or might not fit into it."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:111
#, no-wrap
msgid "Kernel Preemption and Critical Sections"
msgstr ""

#. type: Title ====
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:113
#, no-wrap
msgid "Kernel Preemption in a Nutshell"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:116
msgid ""
"Kernel preemption is fairly simple. The basic idea is that a CPU should "
"always be doing the highest priority work available. Well, that is the ideal "
"at least. There are a couple of cases where the expense of achieving the "
"ideal is not worth being perfect."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:118
msgid ""
"Implementing full kernel preemption is very straightforward: when you "
"schedule a thread to be executed by putting it on a run queue, you check to "
"see if its priority is higher than the currently executing thread. If so, "
"you initiate a context switch to that thread."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:120
msgid ""
"While locks can protect most data in the case of a preemption, not all of "
"the kernel is preemption safe. For example, if a thread holding a spin mutex "
"preempted and the new thread attempts to grab the same spin mutex, the new "
"thread may spin forever as the interrupted thread may never get a chance to "
"execute. Also, some code such as the code to assign an address space number "
"for a process during `exec` on the Alpha needs to not be preempted as it "
"supports the actual context switch code. Preemption is disabled for these "
"code sections by using a critical section."
msgstr ""

#. type: Title ====
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:121
#, no-wrap
msgid "Critical Sections"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:124
msgid ""
"The responsibility of the critical section API is to prevent context "
"switches inside of a critical section. With a fully preemptive kernel, every "
"`setrunqueue` of a thread other than the current thread is a preemption "
"point. One implementation is for `critical_enter` to set a per-thread flag "
"that is cleared by its counterpart. If `setrunqueue` is called with this "
"flag set, it does not preempt regardless of the priority of the new thread "
"relative to the current thread. However, since critical sections are used in "
"spin mutexes to prevent context switches and multiple spin mutexes can be "
"acquired, the critical section API must support nesting. For this reason the "
"current implementation uses a nesting count instead of a single per-thread "
"flag."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:126
msgid ""
"In order to minimize latency, preemptions inside of a critical section are "
"deferred rather than dropped. If a thread that would normally be preempted "
"to is made runnable while the current thread is in a critical section, then "
"a per-thread flag is set to indicate that there is a pending preemption. "
"When the outermost critical section is exited, the flag is checked. If the "
"flag is set, then the current thread is preempted to allow the higher "
"priority thread to run."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:128
msgid ""
"Interrupts pose a problem with regards to spin mutexes. If a low-level "
"interrupt handler needs a lock, it needs to not interrupt any code needing "
"that lock to avoid possible data structure corruption. Currently, providing "
"this mechanism is piggybacked onto critical section API by means of the "
"`cpu_critical_enter` and `cpu_critical_exit` functions. Currently this API "
"disables and re-enables interrupts on all of FreeBSD's current platforms. "
"This approach may not be purely optimal, but it is simple to understand and "
"simple to get right. Theoretically, this second API need only be used for "
"spin mutexes that are used in primary interrupt context. However, to make "
"the code simpler, it is used for all spin mutexes and even all critical "
"sections. It may be desirable to split out the MD API from the MI API and "
"only use it in conjunction with the MI API in the spin mutex implementation. "
"If this approach is taken, then the MD API likely would need a rename to "
"show that it is a separate API."
msgstr ""

#. type: Title ====
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:129
#, no-wrap
msgid "Design Tradeoffs"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:132
msgid ""
"As mentioned earlier, a couple of trade-offs have been made to sacrifice "
"cases where perfect preemption may not always provide the best performance."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:134
msgid ""
"The first trade-off is that the preemption code does not take other CPUs "
"into account. Suppose we have a two CPU's A and B with the priority of A's "
"thread as 4 and the priority of B's thread as 2. If CPU B makes a thread "
"with priority 1 runnable, then in theory, we want CPU A to switch to the new "
"thread so that we will be running the two highest priority runnable threads. "
"However, the cost of determining which CPU to enforce a preemption on as "
"well as actually signaling that CPU via an IPI along with the "
"synchronization that would be required would be enormous. Thus, the current "
"code would instead force CPU B to switch to the higher priority thread. Note "
"that this still puts the system in a better position as CPU B is executing a "
"thread of priority 1 rather than a thread of priority 2."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:136
msgid ""
"The second trade-off limits immediate kernel preemption to real-time "
"priority kernel threads. In the simple case of preemption defined above, a "
"thread is always preempted immediately (or as soon as a critical section is "
"exited) if a higher priority thread is made runnable. However, many threads "
"executing in the kernel only execute in a kernel context for a short time "
"before either blocking or returning to userland. Thus, if the kernel "
"preempts these threads to run another non-realtime kernel thread, the kernel "
"may switch out the executing thread just before it is about to sleep or "
"execute. The cache on the CPU must then adjust to the new thread. When the "
"kernel returns to the preempted thread, it must refill all the cache "
"information that was lost. In addition, two extra context switches are "
"performed that could be avoided if the kernel deferred the preemption until "
"the first thread blocked or returned to userland. Thus, by default, the "
"preemption code will only preempt immediately if the higher priority thread "
"is a real-time priority thread."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:138
msgid ""
"Turning on full kernel preemption for all kernel threads has value as a "
"debugging aid since it exposes more race conditions. It is especially useful "
"on UP systems were many races are hard to simulate otherwise. Thus, there is "
"a kernel option `FULL_PREEMPTION` to enable preemption for all kernel "
"threads that can be used for debugging purposes."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:139
#, no-wrap
msgid "Thread Migration"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:142
msgid ""
"Simply put, a thread migrates when it moves from one CPU to another. In a "
"non-preemptive kernel this can only happen at well-defined points such as "
"when calling `msleep` or returning to userland. However, in the preemptive "
"kernel, an interrupt can force a preemption and possible migration at any "
"time. This can have negative affects on per-CPU data since with the "
"exception of `curthread` and `curpcb` the data can change whenever you "
"migrate. Since you can potentially migrate at any time this renders "
"unprotected per-CPU data access rather useless. Thus it is desirable to be "
"able to disable migration for sections of code that need per-CPU data to be "
"stable."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:144
msgid ""
"Critical sections currently prevent migration since they do not allow "
"context switches. However, this may be too strong of a requirement to "
"enforce in some cases since a critical section also effectively blocks "
"interrupt threads on the current processor. As a result, another API has "
"been provided to allow the current thread to indicate that if it preempted "
"it should not migrate to another CPU."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:146
msgid ""
"This API is known as thread pinning and is provided by the scheduler. The "
"API consists of two functions: `sched_pin` and `sched_unpin`. These "
"functions manage a per-thread nesting count `td_pinned`. A thread is pinned "
"when its nesting count is greater than zero and a thread starts off unpinned "
"with a nesting count of zero. Each scheduler implementation is required to "
"ensure that pinned threads are only executed on the CPU that they were "
"executing on when the `sched_pin` was first called. Since the nesting count "
"is only written to by the thread itself and is only read by other threads "
"when the pinned thread is not executing but while `sched_lock` is held, then "
"`td_pinned` does not need any locking. The `sched_pin` function increments "
"the nesting count and `sched_unpin` decrements the nesting count. Note that "
"these functions only operate on the current thread and bind the current "
"thread to the CPU it is executing on at the time. To bind an arbitrary "
"thread to a specific CPU, the `sched_bind` and `sched_unbind` functions "
"should be used instead."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:147
#, no-wrap
msgid "Callouts"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:150
msgid ""
"The `timeout` kernel facility permits kernel services to register functions "
"for execution as part of the `softclock` software interrupt. Events are "
"scheduled based on a desired number of clock ticks, and callbacks to the "
"consumer-provided function will occur at approximately the right time."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:152
msgid ""
"The global list of pending timeout events is protected by a global spin "
"mutex, `callout_lock`; all access to the timeout list must be performed with "
"this mutex held. When `softclock` is woken up, it scans the list of pending "
"timeouts for those that should fire. In order to avoid lock order reversal, "
"the `softclock` thread will release the `callout_lock` mutex when invoking "
"the provided `timeout` callback function. If the `CALLOUT_MPSAFE` flag was "
"not set during registration, then Giant will be grabbed before invoking the "
"callout, and then released afterwards. The `callout_lock` mutex will be re-"
"grabbed before proceeding. The `softclock` code is careful to leave the list "
"in a consistent state while releasing the mutex. If `DIAGNOSTIC` is enabled, "
"then the time taken to execute each function is measured, and a warning is "
"generated if it exceeds a threshold."
msgstr ""

#. type: Title ==
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:154
#, no-wrap
msgid "Specific Locking Strategies"
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:156
#, no-wrap
msgid "Credentials"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:159
msgid ""
"`struct ucred` is the kernel's internal credential structure, and is "
"generally used as the basis for process-driven access control within the "
"kernel. BSD-derived systems use a \"copy-on-write\" model for credential "
"data: multiple references may exist for a credential structure, and when a "
"change needs to be made, the structure is duplicated, modified, and then the "
"reference replaced. Due to wide-spread caching of the credential to "
"implement access control on open, this results in substantial memory "
"savings. With a move to fine-grained SMP, this model also saves "
"substantially on locking operations by requiring that modification only "
"occur on an unshared credential, avoiding the need for explicit "
"synchronization when consuming a known-shared credential."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:161
msgid ""
"Credential structures with a single reference are considered mutable; shared "
"credential structures must not be modified or a race condition is risked. A "
"mutex, `cr_mtxp` protects the reference count of `struct ucred` so as to "
"maintain consistency. Any use of the structure requires a valid reference "
"for the duration of the use, or the structure may be released out from under "
"the illegitimate consumer."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:163
msgid ""
"The `struct ucred` mutex is a leaf mutex and is implemented via a mutex pool "
"for performance reasons."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:165
msgid ""
"Usually, credentials are used in a read-only manner for access control "
"decisions, and in this case `td_ucred` is generally preferred because it "
"requires no locking. When a process' credential is updated the `proc` lock "
"must be held across the check and update operations thus avoid races. The "
"process credential `p_ucred` must be used for check and update operations to "
"prevent time-of-check, time-of-use races."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:167
msgid ""
"If system call invocations will perform access control after an update to "
"the process credential, the value of `td_ucred` must also be refreshed to "
"the current process value. This will prevent use of a stale credential "
"following a change. The kernel automatically refreshes the `td_ucred` "
"pointer in the thread structure from the process `p_ucred` whenever a "
"process enters the kernel, permitting use of a fresh credential for kernel "
"access control."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:168
#, no-wrap
msgid "File Descriptors and File Descriptor Tables"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:171
msgid "Details to follow."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:172
#, no-wrap
msgid "Jail Structures"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:175
msgid ""
"`struct prison` stores administrative details pertinent to the maintenance "
"of jails created using the man:jail[2] API. This includes the per-jail "
"hostname, IP address, and related settings. This structure is reference-"
"counted since pointers to instances of the structure are shared by many "
"credential structures. A single mutex, `pr_mtx` protects read and write "
"access to the reference count and all mutable variables inside the struct "
"jail. Some variables are set only when the jail is created, and a valid "
"reference to the `struct prison` is sufficient to read these values. The "
"precise locking of each entry is documented via comments in [.filename]#sys/"
"jail.h#."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:176
#, no-wrap
msgid "MAC Framework"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:179
msgid ""
"The TrustedBSD MAC Framework maintains data in a variety of kernel objects, "
"in the form of `struct label`. In general, labels in kernel objects are "
"protected by the same lock as the remainder of the kernel object. For "
"example, the `v_label` label in `struct vnode` is protected by the vnode "
"lock on the vnode."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:181
msgid ""
"In addition to labels maintained in standard kernel objects, the MAC "
"Framework also maintains a list of registered and active policies. The "
"policy list is protected by a global mutex (`mac_policy_list_lock`) and a "
"busy count (also protected by the mutex). Since many access control checks "
"may occur in parallel, entry to the framework for a read-only access to the "
"policy list requires holding the mutex while incrementing (and later "
"decrementing) the busy count. The mutex need not be held for the duration of "
"the MAC entry operation--some operations, such as label operations on file "
"system objects--are long-lived. To modify the policy list, such as during "
"policy registration and de-registration, the mutex must be held and the "
"reference count must be zero, to prevent modification of the list while it "
"is in use."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:183
msgid ""
"A condition variable, `mac_policy_list_not_busy`, is available to threads "
"that need to wait for the list to become unbusy, but this condition variable "
"must only be waited on if the caller is holding no other locks, or a lock "
"order violation may be possible. The busy count, in effect, acts as a form "
"of shared/exclusive lock over access to the framework: the difference is "
"that, unlike with an sx lock, consumers waiting for the list to become "
"unbusy may be starved, rather than permitting lock order problems with "
"regards to the busy count and other locks that may be held on entry to (or "
"inside) the MAC Framework."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:184
#, no-wrap
msgid "Modules"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:187
msgid ""
"For the module subsystem there exists a single lock that is used to protect "
"the shared data. This lock is a shared/exclusive (SX) lock and has a good "
"chance of needing to be acquired (shared or exclusively), therefore there "
"are a few macros that have been added to make access to the lock more easy. "
"These macros can be located in [.filename]#sys/module.h# and are quite basic "
"in terms of usage. The main structures protected under this lock are the "
"`module_t` structures (when shared) and the global `modulelist_t` structure, "
"modules. One should review the related source code in [.filename]#kern/"
"kern_module.c# to further understand the locking strategy."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:188
#, no-wrap
msgid "Newbus Device Tree"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:191
msgid ""
"The newbus system will have one sx lock. Readers will hold a shared (read) "
"lock (man:sx_slock[9]) and writers will hold an exclusive (write) lock "
"(man:sx_xlock[9]). Internal functions will not do locking at all. Externally "
"visible ones will lock as needed. Those items that do not matter if the race "
"is won or lost will not be locked, since they tend to be read all over the "
"place (e.g., man:device_get_softc[9]). There will be relatively few changes "
"to the newbus data structures, so a single lock should be sufficient and not "
"impose a performance penalty."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:192
#, no-wrap
msgid "Pipes"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:195
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:217
msgid "..."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:196
#, no-wrap
msgid "Processes and Threads"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:199
msgid "process hierarchy"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:201
msgid "proc locks, references"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:203
msgid ""
"thread-specific copies of proc entries to freeze during system calls, "
"including td_ucred"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:205
msgid "inter-process operations"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:207
msgid "process groups and sessions"
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:208
#, no-wrap
msgid "Scheduler"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:211
msgid ""
"Lots of references to `sched_lock` and notes pointing at specific primitives "
"and related magic elsewhere in the document."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:212
#, no-wrap
msgid "Select and Poll"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:215
msgid ""
"The `select` and `poll` functions permit threads to block waiting on events "
"on file descriptors--most frequently, whether or not the file descriptors "
"are readable or writable."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:218
#, no-wrap
msgid "SIGIO"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:221
msgid ""
"The SIGIO service permits processes to request the delivery of a SIGIO "
"signal to its process group when the read/write status of specified file "
"descriptors changes. At most one process or process group is permitted to "
"register for SIGIO from any given kernel object, and that process or group "
"is referred to as the owner. Each object supporting SIGIO registration "
"contains pointer field that is `NULL` if the object is not registered, or "
"points to a `struct sigio` describing the registration. This field is "
"protected by a global mutex, `sigio_lock`. Callers to SIGIO maintenance "
"functions must pass in this field \"by reference\" so that local register "
"copies of the field are not made when unprotected by the lock."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:223
msgid ""
"One `struct sigio` is allocated for each registered object associated with "
"any process or process group, and contains back-pointers to the object, "
"owner, signal information, a credential, and the general disposition of the "
"registration. Each process or progress group contains a list of registered "
"`struct sigio` structures, `p_sigiolst` for processes, and `pg_sigiolst` for "
"process groups. These lists are protected by the process or process group "
"locks respectively. Most fields in each `struct sigio` are constant for the "
"duration of the registration, with the exception of the `sio_pgsigio` field "
"which links the `struct sigio` into the process or process group list. "
"Developers implementing new kernel objects supporting SIGIO will, in "
"general, want to avoid holding structure locks while invoking SIGIO "
"supporting functions, such as `fsetown` or `funsetown` to avoid defining a "
"lock order between structure locks and the global SIGIO lock. This is "
"generally possible through use of an elevated reference count on the "
"structure, such as reliance on a file descriptor reference to a pipe during "
"a pipe operation."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:224
#, no-wrap
msgid "Sysctl"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:227
msgid ""
"The `sysctl` MIB service is invoked from both within the kernel and from "
"userland applications using a system call. At least two issues are raised in "
"locking: first, the protection of the structures maintaining the namespace, "
"and second, interactions with kernel variables and functions that are "
"accessed by the sysctl interface. Since sysctl permits the direct export "
"(and modification) of kernel statistics and configuration parameters, the "
"sysctl mechanism must become aware of appropriate locking semantics for "
"those variables. Currently, sysctl makes use of a single global sx lock to "
"serialize use of `sysctl`; however, it is assumed to operate under Giant and "
"other protections are not provided. The remainder of this section speculates "
"on locking and semantic changes to sysctl."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:229
msgid ""
"Need to change the order of operations for sysctl's that update values from "
"read old, copyin and copyout, write new to copyin, lock, read old and write "
"new, unlock, copyout. Normal sysctl's that just copyout the old value and "
"set a new value that they copyin may still be able to follow the old model. "
"However, it may be cleaner to use the second model for all of the sysctl "
"handlers to avoid lock operations."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:231
msgid ""
"To allow for the common case, a sysctl could embed a pointer to a mutex in "
"the SYSCTL_FOO macros and in the struct. This would work for most sysctl's. "
"For values protected by sx locks, spin mutexes, or other locking strategies "
"besides a single sleep mutex, SYSCTL_PROC nodes could be used to get the "
"locking right."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:232
#, no-wrap
msgid "Taskqueue"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:235
msgid ""
"The taskqueue's interface has two basic locks associated with it in order to "
"protect the related shared data. The `taskqueue_queues_mutex` is meant to "
"serve as a lock to protect the `taskqueue_queues` TAILQ. The other mutex "
"lock associated with this system is the one in the `struct taskqueue` data "
"structure. The use of the synchronization primitive here is to protect the "
"integrity of the data in the `struct taskqueue`. It should be noted that "
"there are no separate macros to assist the user in locking down his/her own "
"work since these locks are most likely not going to be used outside of "
"[.filename]#kern/subr_taskqueue.c#."
msgstr ""

#. type: Title ==
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:237
#, no-wrap
msgid "Implementation Notes"
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:239
#, no-wrap
msgid "Sleep Queues"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:242
msgid ""
"A sleep queue is a structure that holds the list of threads asleep on a wait "
"channel. Each thread that is not asleep on a wait channel carries a sleep "
"queue structure around with it. When a thread blocks on a wait channel, it "
"donates its sleep queue structure to that wait channel. Sleep queues "
"associated with a wait channel are stored in a hash table."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:244
msgid ""
"The sleep queue hash table holds sleep queues for wait channels that have at "
"least one blocked thread. Each entry in the hash table is called a "
"sleepqueue chain. The chain contains a linked list of sleep queues and a "
"spin mutex. The spin mutex protects the list of sleep queues as well as the "
"contents of the sleep queue structures on the list. Only one sleep queue is "
"associated with a given wait channel. If multiple threads block on a wait "
"channel than the sleep queues associated with all but the first thread are "
"stored on a list of free sleep queues in the master sleep queue. When a "
"thread is removed from the sleep queue it is given one of the sleep queue "
"structures from the master queue's free list if it is not the only thread "
"asleep on the queue. The last thread is given the master sleep queue when it "
"is resumed. Since threads may be removed from the sleep queue in a different "
"order than they are added, a thread may depart from a sleep queue with a "
"different sleep queue structure than the one it arrived with."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:246
msgid ""
"The `sleepq_lock` function locks the spin mutex of the sleep queue chain "
"that maps to a specific wait channel. The `sleepq_lookup` function looks in "
"the hash table for the master sleep queue associated with a given wait "
"channel. If no master sleep queue is found, it returns `NULL`. The "
"`sleepq_release` function unlocks the spin mutex associated with a given "
"wait channel."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:248
msgid ""
"A thread is added to a sleep queue via the `sleepq_add`. This function "
"accepts the wait channel, a pointer to the mutex that protects the wait "
"channel, a wait message description string, and a mask of flags. The sleep "
"queue chain should be locked via `sleepq_lock` before this function is "
"called. If no mutex protects the wait channel (or it is protected by Giant), "
"then the mutex pointer argument should be `NULL`. The flags argument "
"contains a type field that indicates the kind of sleep queue that the thread "
"is being added to and a flag to indicate if the sleep is interruptible "
"(`SLEEPQ_INTERRUPTIBLE`). Currently there are only two types of sleep "
"queues: traditional sleep queues managed via the `msleep` and `wakeup` "
"functions (`SLEEPQ_MSLEEP`) and condition variable sleep queues "
"(`SLEEPQ_CONDVAR`). The sleep queue type and lock pointer argument are used "
"solely for internal assertion checking. Code that calls `sleepq_add` should "
"explicitly unlock any interlock protecting the wait channel after the "
"associated sleepqueue chain has been locked via `sleepq_lock` and before "
"blocking on the sleep queue via one of the waiting functions."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:250
msgid ""
"A timeout for a sleep is set by invoking `sleepq_set_timeout`. The function "
"accepts the wait channel and the timeout time as a relative tick count as "
"its arguments. If a sleep should be interrupted by arriving signals, the "
"`sleepq_catch_signals` function should be called as well. This function "
"accepts the wait channel as its only parameter. If there is already a signal "
"pending for this thread, then `sleepq_catch_signals` will return a signal "
"number; otherwise, it will return 0."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:252
msgid ""
"Once a thread has been added to a sleep queue, it blocks using one of the "
"`sleepq_wait` functions. There are four wait functions depending on whether "
"or not the caller wishes to use a timeout or have the sleep aborted by "
"caught signals or an interrupt from the userland thread scheduler. The "
"`sleepq_wait` function simply waits until the current thread is explicitly "
"resumed by one of the wakeup functions. The `sleepq_timedwait` function "
"waits until either the thread is explicitly resumed or the timeout set by an "
"earlier call to `sleepq_set_timeout` expires. The `sleepq_wait_sig` function "
"waits until either the thread is explicitly resumed or its sleep is aborted. "
"The `sleepq_timedwait_sig` function waits until either the thread is "
"explicitly resumed, the timeout set by an earlier call to "
"`sleepq_set_timeout` expires, or the thread's sleep is aborted. All of the "
"wait functions accept the wait channel as their first parameter. In "
"addition, the `sleepq_timedwait_sig` function accepts a second boolean "
"parameter to indicate if the earlier call to `sleepq_catch_signals` found a "
"pending signal."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:254
msgid ""
"If the thread is explicitly resumed or is aborted by a signal, then a value "
"of zero is returned by the wait function to indicate a successful sleep. If "
"the thread is resumed by either a timeout or an interrupt from the userland "
"thread scheduler then an appropriate errno value is returned instead. Note "
"that since `sleepq_wait` can only return 0 it does not return anything and "
"the caller should assume a successful sleep. Also, if a thread's sleep times "
"out and is aborted simultaneously then `sleepq_timedwait_sig` will return an "
"error indicating that a timeout occurred. If an error value of 0 is returned "
"and either `sleepq_wait_sig` or `sleepq_timedwait_sig` was used to block, "
"then the function `sleepq_calc_signal_retval` should be called to check for "
"any pending signals and calculate an appropriate return value if any are "
"found. The signal number returned by the earlier call to "
"`sleepq_catch_signals` should be passed as the sole argument to "
"`sleepq_calc_signal_retval`."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:256
msgid ""
"Threads asleep on a wait channel are explicitly resumed by the "
"`sleepq_broadcast` and `sleepq_signal` functions. Both functions accept the "
"wait channel from which to resume threads, a priority to raise resumed "
"threads to, and a flags argument to indicate which type of sleep queue is "
"being resumed. The priority argument is treated as a minimum priority. If a "
"thread being resumed already has a higher priority (numerically lower) than "
"the priority argument then its priority is not adjusted. The flags argument "
"is used for internal assertions to ensure that sleep queues are not being "
"treated as the wrong type. For example, the condition variable functions "
"should not resume threads on a traditional sleep queue. The "
"`sleepq_broadcast` function resumes all threads that are blocked on the "
"specified wait channel while `sleepq_signal` only resumes the highest "
"priority thread blocked on the wait channel. The sleep queue chain should "
"first be locked via the `sleepq_lock` function before calling these "
"functions."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:258
msgid ""
"A sleeping thread may have its sleep interrupted by calling the "
"`sleepq_abort` function. This function must be called with `sched_lock` held "
"and the thread must be queued on a sleep queue. A thread may also be removed "
"from a specific sleep queue via the `sleepq_remove` function. This function "
"accepts both a thread and a wait channel as an argument and only awakens the "
"thread if it is on the sleep queue for the specified wait channel. If the "
"thread is not on a sleep queue or it is on a sleep queue for a different "
"wait channel, then this function does nothing."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:259
#, no-wrap
msgid "Turnstiles"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:262
msgid "Compare/contrast with sleep queues."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:264
msgid "Lookup/wait/release. - Describe TDF_TSNOBLOCK race."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:266
msgid "Priority propagation."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:267
#, no-wrap
msgid "Details of the Mutex Implementation"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:270
msgid ""
"Should we require mutexes to be owned for mtx_destroy() since we can not "
"safely assert that they are unowned by anyone else otherwise?"
msgstr ""

#. type: Title ====
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:271
#, no-wrap
msgid "Spin Mutexes"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:274
msgid "Use a critical section..."
msgstr ""

#. type: Title ====
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:275
#, no-wrap
msgid "Sleep Mutexes"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:278
msgid "Describe the races with contested mutexes"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:280
msgid ""
"Why it is safe to read mtx_lock of a contested mutex when holding the "
"turnstile chain lock."
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:281
#, no-wrap
msgid "Witness"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:284
msgid "What does it do"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:286
msgid "How does it work"
msgstr ""

#. type: Title ==
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:288
#, no-wrap
msgid "Miscellaneous Topics"
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:290
#, no-wrap
msgid "Interrupt Source and ICU Abstractions"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:293
msgid "struct isrc"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:295
msgid "pic drivers"
msgstr ""

#. type: Title ===
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:296
#, no-wrap
msgid "Other Random Questions/Topics"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:299
msgid "Should we pass an interlock into `sema_wait`?"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:301
msgid "Should we have non-sleepable sx locks?"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:303
msgid "Add some info about proper use of reference counts."
msgstr ""

#. type: Title ==
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:308
#, no-wrap
msgid "Glossary"
msgstr ""

#. type: Labeled list
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:311
#, no-wrap
msgid "atomic"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:313
msgid ""
"An operation is atomic if all of its effects are visible to other CPUs "
"together when the proper access protocol is followed. In the degenerate case "
"are atomic instructions provided directly by machine architectures. At a "
"higher level, if several members of a structure are protected by a lock, "
"then a set of operations are atomic if they are all performed while holding "
"the lock without releasing the lock in between any of the operations."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:315
msgid "See Also operation."
msgstr ""

#. type: Labeled list
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:316
#, no-wrap
msgid "block"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:318
msgid ""
"A thread is blocked when it is waiting on a lock, resource, or condition. "
"Unfortunately this term is a bit overloaded as a result."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:320
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:357
msgid "See Also sleep."
msgstr ""

#. type: Labeled list
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:321
#, no-wrap
msgid "critical section"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:323
msgid ""
"A section of code that is not allowed to be preempted. A critical section is "
"entered and exited using the man:critical_enter[9] API."
msgstr ""

#. type: Labeled list
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:324
#, no-wrap
msgid "MD"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:326
msgid "Machine dependent."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:328
msgid "See Also MI."
msgstr ""

#. type: Labeled list
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:329
#, no-wrap
msgid "memory operation"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:331
msgid "A memory operation reads and/or writes to a memory location."
msgstr ""

#. type: Labeled list
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:332
#, no-wrap
msgid "MI"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:334
msgid "Machine independent."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:336
msgid "See Also MD."
msgstr ""

#. type: Labeled list
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:337
#, no-wrap
msgid "operation"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:339
msgid "See memory operation."
msgstr ""

#. type: Labeled list
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:340
#, no-wrap
msgid "primary interrupt context"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:342
msgid ""
"Primary interrupt context refers to the code that runs when an interrupt "
"occurs. This code can either run an interrupt handler directly or schedule "
"an asynchronous interrupt thread to execute the interrupt handlers for a "
"given interrupt source."
msgstr ""

#. type: Labeled list
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:343
#, no-wrap
msgid "realtime kernel thread"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:345
msgid ""
"A high priority kernel thread. Currently, the only realtime priority kernel "
"threads are interrupt threads."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:347
msgid "See Also thread."
msgstr ""

#. type: Labeled list
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:348
#, no-wrap
msgid "sleep"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:350
msgid ""
"A thread is asleep when it is blocked on a condition variable or a sleep "
"queue via msleep or tsleep."
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:352
msgid "See Also block."
msgstr ""

#. type: Labeled list
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:353
#, no-wrap
msgid "sleepable lock"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:355
msgid ""
"A sleepable lock is a lock that can be held by a thread which is asleep. "
"Lockmgr locks and sx locks are currently the only sleepable locks in "
"FreeBSD. Eventually, some sx locks such as the allproc and proctree locks "
"may become non-sleepable locks."
msgstr ""

#. type: Labeled list
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:358
#, no-wrap
msgid "thread"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:360
msgid ""
"A kernel thread represented by a struct thread. Threads own locks and hold a "
"single execution context."
msgstr ""

#. type: Labeled list
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:361
#, no-wrap
msgid "wait channel"
msgstr ""

#. type: Plain text
#: documentation/content/en/books/arch-handbook/smp/_index.adoc:363
msgid "A kernel virtual address that threads may sleep on."
msgstr ""