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thread_identity.h (11476B)

---

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

namespace absl {
ABSL_NAMESPACE_BEGIN

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.
 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;
 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 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.
 int priority;                // Priority of thread (updated every so often).

 // 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;

 // 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;

 intptr_t readers;  // Number of readers in mutex.

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

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

// The instances of this class are allocated in NewThreadIdentity() with an
// alignment of PerThreadSynch::kAlignment and never destroyed. Initialization
// should happen in OneTimeInitThreadIdentity().
//
// Instances may be reused by new threads - fields should be reset in
// ResetThreadIdentityBetweenReuse().
//
// NOTE: The layout of fields in this structure is critical, please do not
//       add, remove, or modify the field placements without fully auditing the
//       layout.
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 {
   alignas(void*) char data[256];
 } 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. Note that
// darwin does *not* use tcmalloc, so this can catch you if using MallocHooks
// on Apple platforms. Whatever function is calling your MallocHooks will need
// to watch for recursion on Apple platforms.
//
// 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 directly 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 directly 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 directly set
#else
#define ABSL_THREAD_IDENTITY_MODE_USE_CPP11 2
#endif

#ifdef ABSL_THREAD_IDENTITY_MODE
#error ABSL_THREAD_IDENTITY_MODE cannot be directly 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 defined(__APPLE__) && defined(ABSL_HAVE_THREAD_LOCAL)
#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

#if ABSL_PER_THREAD_TLS
ABSL_CONST_INIT extern ABSL_PER_THREAD_TLS_KEYWORD ThreadIdentity*
   thread_identity_ptr;
#elif defined(ABSL_HAVE_THREAD_LOCAL)
ABSL_CONST_INIT extern thread_local ThreadIdentity* thread_identity_ptr;
#else
#error Thread-local storage not detected on this platform
#endif

// thread_local variables cannot be in headers exposed by DLLs or in certain
// build configurations on Apple platforms. However, it is important for
// performance reasons in general that `CurrentThreadIdentityIfPresent` be
// inlined. In the other cases we opt to have the function not be inlined. Note
// that `CurrentThreadIdentityIfPresent` is declared above so we can exclude
// this entire inline definition.
#if !defined(__APPLE__) && !defined(ABSL_BUILD_DLL) && \
   !defined(ABSL_CONSUME_DLL)
#define ABSL_INTERNAL_INLINE_CURRENT_THREAD_IDENTITY_IF_PRESENT 1
#endif

#ifdef ABSL_INTERNAL_INLINE_CURRENT_THREAD_IDENTITY_IF_PRESENT
inline ThreadIdentity* CurrentThreadIdentityIfPresent() {
 return thread_identity_ptr;
}
#endif

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

}  // namespace base_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_BASE_INTERNAL_THREAD_IDENTITY_H_