void Mutex::LockSlowLoop()

in absl/synchronization/mutex.cc [1914:2028]

• 110 lines of code
• 26 McCabe index (conditional complexity)

void Mutex::LockSlowLoop(SynchWaitParams *waitp, int flags) {
  SchedulingGuard::ScopedDisable disable_rescheduling;
  int c = 0;
  intptr_t v = mu_.load(std::memory_order_relaxed);
  if ((v & kMuEvent) != 0) {
    PostSynchEvent(this,
         waitp->how == kExclusive?  SYNCH_EV_LOCK: SYNCH_EV_READERLOCK);
  }
  ABSL_RAW_CHECK(
      waitp->thread->waitp == nullptr || waitp->thread->suppress_fatal_errors,
      "detected illegal recursion into Mutex code");
  for (;;) {
    v = mu_.load(std::memory_order_relaxed);
    CheckForMutexCorruption(v, "Lock");
    if ((v & waitp->how->slow_need_zero) == 0) {
      if (mu_.compare_exchange_strong(
              v, (waitp->how->fast_or |
                  (v & zap_desig_waker[flags & kMuHasBlocked])) +
                     waitp->how->fast_add,
              std::memory_order_acquire, std::memory_order_relaxed)) {
        if (waitp->cond == nullptr ||
            EvalConditionAnnotated(waitp->cond, this, true, false,
                                   waitp->how == kShared)) {
          break;  // we timed out, or condition true, so return
        }
        this->UnlockSlow(waitp);  // got lock but condition false
        this->Block(waitp->thread);
        flags |= kMuHasBlocked;
        c = 0;
      }
    } else {                      // need to access waiter list
      bool dowait = false;
      if ((v & (kMuSpin|kMuWait)) == 0) {   // no waiters
        // This thread tries to become the one and only waiter.
        PerThreadSynch *new_h = Enqueue(nullptr, waitp, v, flags);
        intptr_t nv = (v & zap_desig_waker[flags & kMuHasBlocked] & kMuLow) |
                      kMuWait;
        ABSL_RAW_CHECK(new_h != nullptr, "Enqueue to empty list failed");
        if (waitp->how == kExclusive && (v & kMuReader) != 0) {
          nv |= kMuWrWait;
        }
        if (mu_.compare_exchange_strong(
                v, reinterpret_cast<intptr_t>(new_h) | nv,
                std::memory_order_release, std::memory_order_relaxed)) {
          dowait = true;
        } else {            // attempted Enqueue() failed
          // zero out the waitp field set by Enqueue()
          waitp->thread->waitp = nullptr;
        }
      } else if ((v & waitp->how->slow_inc_need_zero &
                  ignore_waiting_writers[flags & kMuHasBlocked]) == 0) {
        // This is a reader that needs to increment the reader count,
        // but the count is currently held in the last waiter.
        if (mu_.compare_exchange_strong(
                v, (v & zap_desig_waker[flags & kMuHasBlocked]) | kMuSpin |
                       kMuReader,
                std::memory_order_acquire, std::memory_order_relaxed)) {
          PerThreadSynch *h = GetPerThreadSynch(v);
          h->readers += kMuOne;       // inc reader count in waiter
          do {                        // release spinlock
            v = mu_.load(std::memory_order_relaxed);
          } while (!mu_.compare_exchange_weak(v, (v & ~kMuSpin) | kMuReader,
                                              std::memory_order_release,
                                              std::memory_order_relaxed));
          if (waitp->cond == nullptr ||
              EvalConditionAnnotated(waitp->cond, this, true, false,
                                     waitp->how == kShared)) {
            break;  // we timed out, or condition true, so return
          }
          this->UnlockSlow(waitp);           // got lock but condition false
          this->Block(waitp->thread);
          flags |= kMuHasBlocked;
          c = 0;
        }
      } else if ((v & kMuSpin) == 0 &&  // attempt to queue ourselves
                 mu_.compare_exchange_strong(
                     v, (v & zap_desig_waker[flags & kMuHasBlocked]) | kMuSpin |
                            kMuWait,
                     std::memory_order_acquire, std::memory_order_relaxed)) {
        PerThreadSynch *h = GetPerThreadSynch(v);
        PerThreadSynch *new_h = Enqueue(h, waitp, v, flags);
        intptr_t wr_wait = 0;
        ABSL_RAW_CHECK(new_h != nullptr, "Enqueue to list failed");
        if (waitp->how == kExclusive && (v & kMuReader) != 0) {
          wr_wait = kMuWrWait;      // give priority to a waiting writer
        }
        do {                        // release spinlock
          v = mu_.load(std::memory_order_relaxed);
        } while (!mu_.compare_exchange_weak(
            v, (v & (kMuLow & ~kMuSpin)) | kMuWait | wr_wait |
            reinterpret_cast<intptr_t>(new_h),
            std::memory_order_release, std::memory_order_relaxed));
        dowait = true;
      }
      if (dowait) {
        this->Block(waitp->thread);  // wait until removed from list or timeout
        flags |= kMuHasBlocked;
        c = 0;
      }
    }
    ABSL_RAW_CHECK(
        waitp->thread->waitp == nullptr || waitp->thread->suppress_fatal_errors,
        "detected illegal recursion into Mutex code");
    // delay, then try again
    c = synchronization_internal::MutexDelay(c, GENTLE);
  }
  ABSL_RAW_CHECK(
      waitp->thread->waitp == nullptr || waitp->thread->suppress_fatal_errors,
      "detected illegal recursion into Mutex code");
  if ((v & kMuEvent) != 0) {
    PostSynchEvent(this,
                   waitp->how == kExclusive? SYNCH_EV_LOCK_RETURNING :
                                      SYNCH_EV_READERLOCK_RETURNING);
  }
}