thread_identity.h

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// Copyright 2017 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// Each active thread has an ThreadIdentity that may represent the thread in
// various level interfaces.  ThreadIdentity objects are never deallocated.
// When a thread terminates, its ThreadIdentity object may be reused for a
// thread created later.

#ifndef ABSL_BASE_INTERNAL_THREAD_IDENTITY_H_
#define ABSL_BASE_INTERNAL_THREAD_IDENTITY_H_

#ifndef _WIN32
#include <pthread.h>
// Defines __GOOGLE_GRTE_VERSION__ (via glibc-specific features.h) when
// supported.
#include <unistd.h>
#endif

#include <atomic>
#include <cstdint>

#include "absl/base/internal/per_thread_tls.h"

namespace absl {

struct SynchLocksHeld;
struct SynchWaitParams;

namespace base_internal {

class SpinLock;
struct ThreadIdentity;

// Used by the implementation of absl::Mutex and absl::CondVar.
struct PerThreadSynch {
  // The internal representation of absl::Mutex and absl::CondVar rely
  // on the alignment of PerThreadSynch. Both store the address of the
  // PerThreadSynch in the high-order bits of their internal state,
  // which means the low kLowZeroBits of the address of PerThreadSynch
  // must be zero.
  static constexpr int kLowZeroBits = 8;
  static constexpr int kAlignment = 1 << kLowZeroBits;

  // Returns the associated ThreadIdentity.
  // This can be implemented as a cast because we guarantee
  // PerThreadSynch is the first element of ThreadIdentity.
  ThreadIdentity* thread_identity() {
    return reinterpret_cast<ThreadIdentity*>(this);
  }

  PerThreadSynch *next;  // Circular waiter queue; initialized to 0.
  PerThreadSynch *skip;  // If non-zero, all entries in Mutex queue
                         // up to and including "skip" have same
                         // condition as this, and will be woken later
  bool may_skip;         // if false while on mutex queue, a mutex unlocker
                         // is using this PerThreadSynch as a terminator.  Its
                         // skip field must not be filled in because the loop
                         // might then skip over the terminator.

  // The wait parameters of the current wait.  waitp is null if the
  // thread is not waiting. Transitions from null to non-null must
  // occur before the enqueue commit point (state = kQueued in
  // Enqueue() and CondVarEnqueue()). Transitions from non-null to
  // null must occur after the wait is finished (state = kAvailable in
  // Mutex::Block() and CondVar::WaitCommon()). This field may be
  // changed only by the thread that describes this PerThreadSynch.  A
  // special case is Fer(), which calls Enqueue() on another thread,
  // but with an identical SynchWaitParams pointer, thus leaving the
  // pointer unchanged.
  SynchWaitParams *waitp;

  bool suppress_fatal_errors;  // If true, try to proceed even in the face of
                               // broken invariants.  This is used within fatal
                               // signal handlers to improve the chances of
                               // debug logging information being output
                               // successfully.

  intptr_t readers;     // Number of readers in mutex.
  int priority;         // Priority of thread (updated every so often).

  // When priority will next be read (cycles).
  int64_t next_priority_read_cycles;

  // State values:
  //   kAvailable: This PerThreadSynch is available.
  //   kQueued: This PerThreadSynch is unavailable, it's currently queued on a
  //            Mutex or CondVar waistlist.
  //
  // Transitions from kQueued to kAvailable require a release
  // barrier. This is needed as a waiter may use "state" to
  // independently observe that it's no longer queued.
  //
  // Transitions from kAvailable to kQueued require no barrier, they
  // are externally ordered by the Mutex.
  enum State {
    kAvailable,
    kQueued
  };
  std::atomic<State> state;

  bool maybe_unlocking;  // Valid at head of Mutex waiter queue;
                         // true if UnlockSlow could be searching
                         // for a waiter to wake.  Used for an optimization
                         // in Enqueue().  true is always a valid value.
                         // Can be reset to false when the unlocker or any
                         // writer releases the lock, or a reader fully releases
                         // the lock.  It may not be set to false by a reader
                         // that decrements the count to non-zero.
                         // protected by mutex spinlock

  bool wake;  // This thread is to be woken from a Mutex.

  // If "x" is on a waiter list for a mutex, "x->cond_waiter" is true iff the
  // waiter is waiting on the mutex as part of a CV Wait or Mutex Await.
  //
  // The value of "x->cond_waiter" is meaningless if "x" is not on a
  // Mutex waiter list.
  bool cond_waiter;

  // Locks held; used during deadlock detection.
  // Allocated in Synch_GetAllLocks() and freed in ReclaimThreadIdentity().
  SynchLocksHeld *all_locks;
};

struct ThreadIdentity {
  // Must be the first member.  The Mutex implementation requires that
  // the PerThreadSynch object associated with each thread is
  // PerThreadSynch::kAlignment aligned.  We provide this alignment on
  // ThreadIdentity itself.
  PerThreadSynch per_thread_synch;

  // Private: Reserved for absl::synchronization_internal::Waiter.
  struct WaiterState {
    char data[128];
  } waiter_state;

  // Used by PerThreadSem::{Get,Set}ThreadBlockedCounter().
  std::atomic<int>* blocked_count_ptr;

  // The following variables are mostly read/written just by the
  // thread itself.  The only exception is that these are read by
  // a ticker thread as a hint.
  std::atomic<int> ticker;      // Tick counter, incremented once per second.
  std::atomic<int> wait_start;  // Ticker value when thread started waiting.
  std::atomic<bool> is_idle;    // Has thread become idle yet?

  ThreadIdentity* next;
};

// Returns the ThreadIdentity object representing the calling thread; guaranteed
// to be unique for its lifetime.  The returned object will remain valid for the
// program's lifetime; although it may be re-assigned to a subsequent thread.
// If one does not exist, return nullptr instead.
//
// Does not malloc(*), and is async-signal safe.
// [*] Technically pthread_setspecific() does malloc on first use; however this
// is handled internally within tcmalloc's initialization already.
//
// New ThreadIdentity objects can be constructed and associated with a thread
// by calling GetOrCreateCurrentThreadIdentity() in per-thread-sem.h.
ThreadIdentity* CurrentThreadIdentityIfPresent();

using ThreadIdentityReclaimerFunction = void (*)(void*);

// Sets the current thread identity to the given value.  'reclaimer' is a
// pointer to the global function for cleaning up instances on thread
// destruction.
void SetCurrentThreadIdentity(ThreadIdentity* identity,
                              ThreadIdentityReclaimerFunction reclaimer);

// Removes the currently associated ThreadIdentity from the running thread.
// This must be called from inside the ThreadIdentityReclaimerFunction, and only
// from that function.
void ClearCurrentThreadIdentity();

// May be chosen at compile time via: -DABSL_FORCE_THREAD_IDENTITY_MODE=<mode
// index>
#ifdef ABSL_THREAD_IDENTITY_MODE_USE_POSIX_SETSPECIFIC
#error ABSL_THREAD_IDENTITY_MODE_USE_POSIX_SETSPECIFIC cannot be direcly set
#else
#define ABSL_THREAD_IDENTITY_MODE_USE_POSIX_SETSPECIFIC 0
#endif

#ifdef ABSL_THREAD_IDENTITY_MODE_USE_TLS
#error ABSL_THREAD_IDENTITY_MODE_USE_TLS cannot be direcly set
#else
#define ABSL_THREAD_IDENTITY_MODE_USE_TLS 1
#endif

#ifdef ABSL_THREAD_IDENTITY_MODE_USE_CPP11
#error ABSL_THREAD_IDENTITY_MODE_USE_CPP11 cannot be direcly set
#else
#define ABSL_THREAD_IDENTITY_MODE_USE_CPP11 2
#endif

#ifdef ABSL_THREAD_IDENTITY_MODE
#error ABSL_THREAD_IDENTITY_MODE cannot be direcly set
#elif defined(ABSL_FORCE_THREAD_IDENTITY_MODE)
#define ABSL_THREAD_IDENTITY_MODE ABSL_FORCE_THREAD_IDENTITY_MODE
#elif defined(_WIN32)
#define ABSL_THREAD_IDENTITY_MODE ABSL_THREAD_IDENTITY_MODE_USE_CPP11
#elif ABSL_PER_THREAD_TLS && defined(__GOOGLE_GRTE_VERSION__) && \
    (__GOOGLE_GRTE_VERSION__ >= 20140228L)
// Support for async-safe TLS was specifically added in GRTEv4.  It's not
// present in the upstream eglibc.
// Note:  Current default for production systems.
#define ABSL_THREAD_IDENTITY_MODE ABSL_THREAD_IDENTITY_MODE_USE_TLS
#else
#define ABSL_THREAD_IDENTITY_MODE \
  ABSL_THREAD_IDENTITY_MODE_USE_POSIX_SETSPECIFIC
#endif

#if ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_TLS || \
    ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_CPP11

extern ABSL_PER_THREAD_TLS_KEYWORD ThreadIdentity* thread_identity_ptr;

inline ThreadIdentity* CurrentThreadIdentityIfPresent() {
  return thread_identity_ptr;
}

#elif ABSL_THREAD_IDENTITY_MODE != \
    ABSL_THREAD_IDENTITY_MODE_USE_POSIX_SETSPECIFIC
#error Unknown ABSL_THREAD_IDENTITY_MODE
#endif

}  // namespace base_internal
}  // namespace absl

#endif  // ABSL_BASE_INTERNAL_THREAD_IDENTITY_H_