os_posix.hpp

/*
 * Copyright (c) 1999, 2021, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 *
 */

#ifndef OS_POSIX_OS_POSIX_HPP
#define OS_POSIX_OS_POSIX_HPP

// File conventions
static const char* file_separator() { return "/"; }
static const char* line_separator() { return "\n"; }
static const char* path_separator() { return ":"; }

class Posix {
  friend class os;

protected:
  static void print_distro_info(outputStream* st);
  static void print_rlimit_info(outputStream* st);
  static void print_uname_info(outputStream* st);
  static void print_libversion_info(outputStream* st);
  static void print_load_average(outputStream* st);
  static void print_uptime_info(outputStream* st);

  // Minimum stack size a thread can be created with (allowing
  // the VM to completely create the thread and enter user code).
  // The initial values exclude any guard pages (by HotSpot or libc).
  // set_minimum_stack_sizes() will add the size required for
  // HotSpot guard pages depending on page size and flag settings.
  // Libc guard pages are never considered by these values.
  static size_t _compiler_thread_min_stack_allowed;
  static size_t _java_thread_min_stack_allowed;
  static size_t _vm_internal_thread_min_stack_allowed;

public:
  static void init(void);  // early initialization - no logging available
  static void init_2(void);// later initialization - logging available

  // Return default stack size for the specified thread type
  static size_t default_stack_size(os::ThreadType thr_type);
  // Check and sets minimum stack sizes
  static jint set_minimum_stack_sizes();
  static size_t get_initial_stack_size(ThreadType thr_type, size_t req_stack_size);

  // Helper function; describes pthread attributes as short string. String is written
  // to buf with len buflen; buf is returned.
  static char* describe_pthread_attr(char* buf, size_t buflen, const pthread_attr_t* attr);

  // A safe implementation of realpath which will not cause a buffer overflow if the resolved path
  //   is longer than PATH_MAX.
  // On success, returns 'outbuf', which now contains the path.
  // On error, it will return NULL and set errno. The content of 'outbuf' is undefined.
  // On truncation error ('outbuf' too small), it will return NULL and set errno to ENAMETOOLONG.
  static char* realpath(const char* filename, char* outbuf, size_t outbuflen);

  // Returns true if given uid is root.
  static bool is_root(uid_t uid);

  // Returns true if given uid is effective or root uid.
  static bool matches_effective_uid_or_root(uid_t uid);

  // Returns true if either given uid is effective uid and given gid is
  // effective gid, or if given uid is root.
  static bool matches_effective_uid_and_gid_or_root(uid_t uid, gid_t gid);

  static void print_umask(outputStream* st, mode_t umsk);

  static void print_user_info(outputStream* st);

  // Set PC into context. Needed for continuation after signal.
  static address ucontext_get_pc(const ucontext_t* ctx);
  static void    ucontext_set_pc(ucontext_t* ctx, address pc);

  static void to_RTC_abstime(timespec* abstime, int64_t millis);

  static bool handle_stack_overflow(JavaThread* thread, address addr, address pc,
                                    const void* ucVoid,
                                    address* stub);
};

/*
 * Crash protection for the JfrSampler thread. Wrap the callback
 * with a sigsetjmp and in case of a SIGSEGV/SIGBUS we siglongjmp
 * back.
 * To be able to use this - don't take locks, don't rely on destructors,
 * don't make OS library calls, don't allocate memory, don't print,
 * don't call code that could leave the heap / memory in an inconsistent state,
 * or anything else where we are not in control if we suddenly jump out.
 */
class ThreadCrashProtection : public StackObj {
public:
  static bool is_crash_protected(Thread* thr) {
    return _crash_protection != NULL && _protected_thread == thr;
  }

  ThreadCrashProtection();
  bool call(os::CrashProtectionCallback& cb);

  static void check_crash_protection(int signal, Thread* thread);
private:
  static Thread* _protected_thread;
  static ThreadCrashProtection* _crash_protection;
  void restore();
  sigjmp_buf _jmpbuf;
};

/*
 * This is the platform-specific implementation underpinning
 * the ParkEvent class, which itself underpins Java-level monitor
 * operations. See park.hpp for details.
 * These event objects are type-stable and immortal - we never delete them.
 * Events are associated with a thread for the lifetime of the thread.
 */
class PlatformEvent : public CHeapObj<mtSynchronizer> {
 private:
  double cachePad[4];        // Increase odds that _mutex is sole occupant of cache line
  volatile int _event;       // Event count/permit: -1, 0 or 1
  volatile int _nParked;     // Indicates if associated thread is blocked: 0 or 1
  pthread_mutex_t _mutex[1]; // Native mutex for locking
  pthread_cond_t  _cond[1];  // Native condition variable for blocking
  double postPad[2];

 protected:       // TODO-FIXME: make dtor private
  ~PlatformEvent() { guarantee(false, "invariant"); } // immortal so can't delete

 public:
  PlatformEvent();
  void park();
  int  park(jlong millis);
  void unpark();

  // Use caution with reset() and fired() -- they may require MEMBARs
  void reset() { _event = 0; }
  int  fired() { return _event; }
};

// JSR166 support
// PlatformParker provides the platform dependent base class for the
// Parker class. It basically provides the internal data structures:
// - mutex and convars
// which are then used directly by the Parker methods defined in the OS
// specific implementation files.
// There is significant overlap between the funcionality supported in the
// combination of Parker+PlatformParker and PlatformEvent (above). If Parker
// were more like ObjectMonitor we could use PlatformEvent in both (with some
// API updates of course). But Parker methods use fastpaths that break that
// level of encapsulation - so combining the two remains a future project.

class PlatformParker {
  NONCOPYABLE(PlatformParker);
 protected:
  enum {
    REL_INDEX = 0,
    ABS_INDEX = 1
  };
  volatile int _counter;
  int _cur_index;  // which cond is in use: -1, 0, 1
  pthread_mutex_t _mutex[1];
  pthread_cond_t  _cond[2]; // one for relative times and one for absolute

 public:
  PlatformParker();
  ~PlatformParker();
};

// Workaround for a bug in macOSX kernel's pthread support (fixed in Mojave?).
// Avoid ever allocating a pthread_mutex_t at the same address as one of our
// former pthread_cond_t, by using freelists of mutexes and condvars.
// Conditional to avoid extra indirection and padding loss on other platforms.
#ifdef __APPLE__
#define PLATFORM_MONITOR_IMPL_INDIRECT 1
#else
#define PLATFORM_MONITOR_IMPL_INDIRECT 0
#endif

// Platform specific implementations that underpin VM Mutex/Monitor classes.
// Note that we use "normal" pthread_mutex_t attributes so that recursive
// locking is not supported, which matches the expected semantics of the
// VM Mutex class.

class PlatformMutex : public CHeapObj<mtSynchronizer> {
#if PLATFORM_MONITOR_IMPL_INDIRECT
  class Mutex : public CHeapObj<mtSynchronizer> {
   public:
    pthread_mutex_t _mutex;
    Mutex* _next;

    Mutex();
    ~Mutex();
  };

  Mutex* _impl;

  static pthread_mutex_t _freelist_lock; // used for mutex and cond freelists
  static Mutex* _mutex_freelist;

 protected:
  class WithFreeListLocked;
  pthread_mutex_t* mutex() { return &(_impl->_mutex); }

 public:
  PlatformMutex();              // Use freelist allocation of impl.
  ~PlatformMutex();

  static void init();           // Initialize the freelist.

#else

  pthread_mutex_t _mutex;

 protected:
  pthread_mutex_t* mutex() { return &_mutex; }

 public:
  static void init() {}         // Nothing needed for the non-indirect case.

  PlatformMutex();
  ~PlatformMutex();

#endif // PLATFORM_MONITOR_IMPL_INDIRECT

 private:
  NONCOPYABLE(PlatformMutex);

 public:
  void lock();
  void unlock();
  bool try_lock();
};

class PlatformMonitor : public PlatformMutex {
#if PLATFORM_MONITOR_IMPL_INDIRECT
  class Cond : public CHeapObj<mtSynchronizer> {
   public:
    pthread_cond_t _cond;
    Cond* _next;

    Cond();
    ~Cond();
  };

  Cond* _impl;

  static Cond* _cond_freelist;

  pthread_cond_t* cond() { return &(_impl->_cond); }

 public:
  PlatformMonitor();            // Use freelist allocation of impl.
  ~PlatformMonitor();

#else

  pthread_cond_t _cond;
  pthread_cond_t* cond() { return &_cond; }

 public:
  PlatformMonitor();
  ~PlatformMonitor();

#endif // PLATFORM_MONITOR_IMPL_INDIRECT

 private:
  NONCOPYABLE(PlatformMonitor);

 public:
  int wait(jlong millis);
  void notify();
  void notify_all();
};

#endif // OS_POSIX_OS_POSIX_HPP