//

// 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.

//

//  Most users requiring mutual exclusion should use Mutex.

//  SpinLock is provided for use in two situations:

//   - for use by Abseil internal code that Mutex itself depends on

//   - for async signal safety (see below)

// SpinLock is async signal safe.  If a spinlock is used within a signal

// handler, all code that acquires the lock must ensure that the signal cannot

// arrive while they are holding the lock.  Typically, this is done by blocking

// the signal.

//

// Threads waiting on a SpinLock may be woken in an arbitrary order.

#ifndef ABSL_BASE_INTERNAL_SPINLOCK_H_

#define ABSL_BASE_INTERNAL_SPINLOCK_H_

#include <atomic>

#include <cstdint>

#include "absl/base/attributes.h"

#include "absl/base/const_init.h"

#include "absl/base/dynamic_annotations.h"

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

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

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

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

#include "absl/base/thread_annotations.h"

namespace absl {

ABSL_NAMESPACE_BEGIN

namespace base_internal {

class ABSL_LOCKABLE SpinLock {

 public:

  SpinLock() : lockword_(kSpinLockCooperative) {

    ABSL_TSAN_MUTEX_CREATE(this, __tsan_mutex_not_static);

  }

  // Constructors that allow non-cooperative spinlocks to be created for use

  // inside thread schedulers.  Normal clients should not use these.

  explicit SpinLock(base_internal::SchedulingMode mode);

  // Constructor for global SpinLock instances.  See absl/base/const_init.h.

  constexpr SpinLock(absl::ConstInitType, base_internal::SchedulingMode mode)

      : lockword_(IsCooperative(mode) ? kSpinLockCooperative : 0) {}

  // For global SpinLock instances prefer trivial destructor when possible.

  // Default but non-trivial destructor in some build configurations causes an

  // extra static initializer.

#ifdef ABSL_INTERNAL_HAVE_TSAN_INTERFACE

  ~SpinLock() { ABSL_TSAN_MUTEX_DESTROY(this, __tsan_mutex_not_static); }

#else

  ~SpinLock() = default;

#endif

  // Acquire this SpinLock.

  inline void Lock() ABSL_EXCLUSIVE_LOCK_FUNCTION() {

    ABSL_TSAN_MUTEX_PRE_LOCK(this, 0);

    if (!TryLockImpl()) {

      SlowLock();

    }

    ABSL_TSAN_MUTEX_POST_LOCK(this, 0, 0);

  }

  // Try to acquire this SpinLock without blocking and return true if the

  // acquisition was successful.  If the lock was not acquired, false is

  // returned.  If this SpinLock is free at the time of the call, TryLock

  // will return true with high probability.

  inline bool TryLock() ABSL_EXCLUSIVE_TRYLOCK_FUNCTION(true) {

    ABSL_TSAN_MUTEX_PRE_LOCK(this, __tsan_mutex_try_lock);

    bool res = TryLockImpl();

    ABSL_TSAN_MUTEX_POST_LOCK(

        this, __tsan_mutex_try_lock | (res ? 0 : __tsan_mutex_try_lock_failed),

        0);

    return res;

  }

  // Release this SpinLock, which must be held by the calling thread.

  inline void Unlock() ABSL_UNLOCK_FUNCTION() {

    ABSL_TSAN_MUTEX_PRE_UNLOCK(this, 0);

    uint32_t lock_value = lockword_.load(std::memory_order_relaxed);

    lock_value = lockword_.exchange(lock_value & kSpinLockCooperative,

                                    std::memory_order_release);

    if ((lock_value & kSpinLockDisabledScheduling) != 0) {

      base_internal::SchedulingGuard::EnableRescheduling(true);

    }

    if ((lock_value & kWaitTimeMask) != 0) {

      // Collect contentionz profile info, and speed the wakeup of any waiter.

      // The wait_cycles value indicates how long this thread spent waiting

      // for the lock.

      SlowUnlock(lock_value);

    }

    ABSL_TSAN_MUTEX_POST_UNLOCK(this, 0);

  }

  // Determine if the lock is held.  When the lock is held by the invoking

  // thread, true will always be returned. Intended to be used as

  // CHECK(lock.IsHeld()).

  inline bool IsHeld() const {

    return (lockword_.load(std::memory_order_relaxed) & kSpinLockHeld) != 0;

  }

  // Return immediately if this thread holds the SpinLock exclusively.

  // Otherwise, report an error by crashing with a diagnostic.

  inline void AssertHeld() const ABSL_ASSERT_EXCLUSIVE_LOCK() {

    if (!IsHeld()) {

      ABSL_RAW_LOG(FATAL, "thread should hold the lock on SpinLock");

    }

  }

 protected:

  // These should not be exported except for testing.

  // Store number of cycles between wait_start_time and wait_end_time in a

  // lock value.

  static uint32_t EncodeWaitCycles(int64_t wait_start_time,

                                   int64_t wait_end_time);

  // Extract number of wait cycles in a lock value.

  static int64_t DecodeWaitCycles(uint32_t lock_value);

  // Provide access to protected method above.  Use for testing only.

  friend struct SpinLockTest;

 private:

  // lockword_ is used to store the following:

  //

  // bit[0] encodes whether a lock is being held.

  // bit[1] encodes whether a lock uses cooperative scheduling.

  // bit[2] encodes whether the current lock holder disabled scheduling when

  //        acquiring the lock. Only set when kSpinLockHeld is also set.

  // bit[3:31] encodes time a lock spent on waiting as a 29-bit unsigned int.

  //        This is set by the lock holder to indicate how long it waited on

  //        the lock before eventually acquiring it. The number of cycles is

  //        encoded as a 29-bit unsigned int, or in the case that the current

  //        holder did not wait but another waiter is queued, the LSB

  //        (kSpinLockSleeper) is set. The implementation does not explicitly

  //        track the number of queued waiters beyond this. It must always be

  //        assumed that waiters may exist if the current holder was required to

  //        queue.

  //

  // Invariant: if the lock is not held, the value is either 0 or

  // kSpinLockCooperative.

  static constexpr uint32_t kSpinLockHeld = 1;

  static constexpr uint32_t kSpinLockCooperative = 2;

  static constexpr uint32_t kSpinLockDisabledScheduling = 4;

  static constexpr uint32_t kSpinLockSleeper = 8;

  // Includes kSpinLockSleeper.

  static constexpr uint32_t kWaitTimeMask =

      ~(kSpinLockHeld | kSpinLockCooperative | kSpinLockDisabledScheduling);

  // Returns true if the provided scheduling mode is cooperative.

  static constexpr bool IsCooperative(

      base_internal::SchedulingMode scheduling_mode) {

    return scheduling_mode == base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL;

  }

  uint32_t TryLockInternal(uint32_t lock_value, uint32_t wait_cycles);

  void SlowLock() ABSL_ATTRIBUTE_COLD;

  void SlowUnlock(uint32_t lock_value) ABSL_ATTRIBUTE_COLD;

  uint32_t SpinLoop();

  inline bool TryLockImpl() {

    uint32_t lock_value = lockword_.load(std::memory_order_relaxed);

    return (TryLockInternal(lock_value, 0) & kSpinLockHeld) == 0;

  }

  std::atomic<uint32_t> lockword_;

  SpinLock(const SpinLock&) = delete;

  SpinLock& operator=(const SpinLock&) = delete;

};

// Corresponding locker object that arranges to acquire a spinlock for

// the duration of a C++ scope.

class ABSL_SCOPED_LOCKABLE SpinLockHolder {

 public:

  inline explicit SpinLockHolder(SpinLock* l) ABSL_EXCLUSIVE_LOCK_FUNCTION(l)

      : lock_(l) {

    l->Lock();

  }

  inline ~SpinLockHolder() ABSL_UNLOCK_FUNCTION() { lock_->Unlock(); }

  SpinLockHolder(const SpinLockHolder&) = delete;

  SpinLockHolder& operator=(const SpinLockHolder&) = delete;

 private:

  SpinLock* lock_;

};

// Register a hook for profiling support.

//

// The function pointer registered here will be called whenever a spinlock is

// contended.  The callback is given an opaque handle to the contended spinlock

// and the number of wait cycles.  This is thread-safe, but only a single

// profiler can be registered.  It is an error to call this function multiple

// times with different arguments.

void RegisterSpinLockProfiler(void (*fn)(const void* lock,

                                         int64_t wait_cycles));

//------------------------------------------------------------------------------

// Public interface ends here.

//------------------------------------------------------------------------------

// If (result & kSpinLockHeld) == 0, then *this was successfully locked.

// Otherwise, returns last observed value for lockword_.

inline uint32_t SpinLock::TryLockInternal(uint32_t lock_value,

                                          uint32_t wait_cycles) {

  if ((lock_value & kSpinLockHeld) != 0) {

    return lock_value;

  }

  uint32_t sched_disabled_bit = 0;

  if ((lock_value & kSpinLockCooperative) == 0) {

    // For non-cooperative locks we must make sure we mark ourselves as

    // non-reschedulable before we attempt to CompareAndSwap.

    if (base_internal::SchedulingGuard::DisableRescheduling()) {

      sched_disabled_bit = kSpinLockDisabledScheduling;

    }

  }

  if (!lockword_.compare_exchange_strong(

          lock_value,

          kSpinLockHeld | lock_value | wait_cycles | sched_disabled_bit,

          std::memory_order_acquire, std::memory_order_relaxed)) {

    base_internal::SchedulingGuard::EnableRescheduling(sched_disabled_bit != 0);

  }

  return lock_value;

}

}  // namespace base_internal

ABSL_NAMESPACE_END

}  // namespace absl

#endif  // ABSL_BASE_INTERNAL_SPINLOCK_H_