Changeset 203350 in webkit

Timestamp:

Jul 18, 2016, 11:32:52 AM (9 years ago)

Author:

fpizlo@apple.com

Message:

WTF::Lock should be fair eventually
​https://bugs.webkit.org/show_bug.cgi?id=159384

Reviewed by Geoffrey Garen.
Source/WTF:

In ​https://webkit.org/blog/6161/locking-in-webkit/ we showed how relaxing the fairness of
locks makes them fast. That post presented lock fairness as a trade-off between two
extremes:

• Barging. A barging lock, like WTF::Lock, releases the lock in unlock() even if there was a thread on the queue. If there was a thread on the queue, the lock is released and that thread is made runnable. That thread may then grab the lock, or some other thread may grab the lock first (it may barge). Usually, the barging thread is the thread that released the lock in the first place. This maximizes throughput but hurts fairness. There is no good theoretical bound on how unfair the lock may become, but empirical data suggests that it's fair enough for the cases we previously measured.

• FIFO. A FIFO lock, like HandoffLock in ToyLocks.h, does not release the lock in unlock() if there is a thread waiting. If there is a thread waiting, unlock() will make that thread runnable and inform it that it now holds the lock. This ensures perfect round-robin fairness and allows us to reason theoretically about how long it may take for a thread to grab the lock. For example, if we know that only N threads are running and each one may contend on a critical section, and each one may hold the lock for at most S seconds, then the time it takes to grab the lock is N * S. Unfortunately, FIFO locks perform very badly in most cases. This is because for the common case of short critical sections, they force a context switch after each critical section if the lock is contended.

This change makes WTF::Lock almost as fair as FIFO while still being as fast as barging.
Thanks to this new algorithm, you can now have both of these things at the same time.

This change makes WTF::Lock eventually fair. We can almost (more on the caveats below)
guarantee that the time it takes to grab a lock is N * max(1ms, S). In other words, critical
sections that are longer than 1ms are always fair. For shorter critical sections, the amount
of time that any thread waits is 1ms times the number of threads. There are some caveats
that arise from our use of randomness, but even then, in the limit as the critical section
length goes to infinity, the lock becomes fair. The corner cases are unlikely to happen; our
experiments show that the lock becomes exactly as fair as a FIFO lock for any critical
section that is 1ms or longer.

The fairness mechanism is broken into two parts. WTF::Lock can now choose to unlock a lock
fairly or unfairly thanks to the new ParkingLot token mechanism. WTF::Lock knows when to use
fair unlocking based on a timeout mechanism in ParkingLot called timeToBeFair.

ParkingLot::unparkOne() and ParkingLot::parkConditionally() can now communicate with each
other via a token. unparkOne() can pass a token, which parkConditionally() will return. This
change also makes parkConditionally() a lot more precise about when it was unparked due to a
call to unparkOne(). If unparkOne() is told that a thread was unparked then this thread is
guaranteed to report that it was unparked rather than timing out, and that thread is
guaranteed to get the token that unparkOne() passed. The token is an intptr_t. We use it as
a boolean variable in WTF::Lock, but you could use it to pass arbitrary data structures. By
default, the token is zero. WTF::Lock's unlock() will pass 1 as the token if it is doing
fair unlocking. In that case, unlock() will not release the lock, and lock() will know that
it holds the lock as soon as parkConditionally() returns. Note that this algorithm relies
on unparkOne() invoking WTF::Lock's callback while the queue lock is held, so that WTF::Lock
can make a decision about unlock strategy and inject a token while it has complete knowledge
over the state of the queue. As such, it's not immediately obvious how to implement this
algorithm on top of futexes. You really need ParkingLot!

WTF::Lock does not use fair unlocking every time. We expose a new API, Lock::unlockFairly(),
which forces the fair unlocking behavior. Additionally, ParkingLot now maintains a
per-bucket stochastic fairness timeout. When the timeout fires, the unparkOne() callback
sees UnparkResult::timeToBeFair = true. This timeout is set to be anywhere from 0ms to 1ms
at random. When a dequeue happens and there are threads that actually get dequeued, we check
if the time since the last unfair unlock (the last time timeToBeFair was set to true) is
more than the timeout amount. If so, then we set timeToBeFair to true and reset the timeout.
This means that in the absence of ParkingLot collisions, unfair unlocking is guaranteed to
happen at least once per millisecond. It will happen at 2 KHz on average. If there are
collisions, then each collision adds one millisecond to the worst case (and 0.5 ms to the
average case). The reason why we don't just use a fixed 1ms timeout is that we want to avoid
resonance. Imagine a program in which some thread acquires a lock at 1 KHz in-phase with the
timeToBeFair timeout. Then this thread would be the benefactor of fairness to the detriment
of everyone else. Randomness ensures that we aren't too fair to any one thread.

Empirically, this is neutral on our major benchmarks like JetStream but it's an enormous
improvement in LockFairnessTest. It's common for an unfair lock (either our BargingLock, the
old WTF::Lock, any of the other futex-based locks that barge, or new os_unfair_lock) to
allow only one thread to hold the lock during a whole second in which each thread is holding
the lock for 1ms at a time. This is because in a barging lock, releasing a lock after
holding it for 1ms and then reacquiring it immediately virtually ensures that none of the
other threads can wake up in time to grab it before it's relocked. But the new WTF::Lock
handles this case like a champ: each thread gets equal turns.

Here's some data. If we launch 10 threads and have each of them run for 1 second while
repeatedly holding a critical section for 1ms, then here's how many times each thread gets
to hold the lock using the old WTF::Lock algorithm:

799, 6, 1, 1, 1, 1, 1, 1, 1, 1

One thread hogged the lock for almost the whole time! With the new WTF::Lock, the lock
becomes totally fair:

80, 79, 79, 79, 79, 79, 79, 80, 80, 79

I don't know of anyone creating such an automatically-fair adaptive lock before, so I think
that this is a pretty awesome advancement to the state of the art!

This change is good for three reasons:

• We do have long critical sections in WebKit and we don't want to have to worry about starvation. This reduces the likelihood that we will see starvation due to our lock strategy.

• I was talking to ggaren about bmalloc's locking needs, and he wanted unlockFairly() or lockFairly() or some moral equivalent for the scavenger thread.

• If we use a WTF::Lock to manage heap access in a multithreaded GC, we'll need the ability to unlock and relock without barging.

• benchmarks/LockFairnessTest.cpp:

(main):

• benchmarks/ToyLocks.h:
• wtf/Condition.h:

(WTF::ConditionBase::waitUntil):
(WTF::ConditionBase::notifyOne):

• wtf/Lock.cpp:

(WTF::LockBase::lockSlow):
(WTF::LockBase::unlockSlow):
(WTF::LockBase::unlockFairlySlow):
(WTF::LockBase::unlockSlowImpl):

• wtf/Lock.h:

(WTF::LockBase::try_lock):
(WTF::LockBase::unlock):
(WTF::LockBase::unlockFairly):
(WTF::LockBase::isHeld):
(WTF::LockBase::isFullyReset):

• wtf/ParkingLot.cpp:

(WTF::ParkingLot::parkConditionallyImpl):
(WTF::ParkingLot::unparkOne):
(WTF::ParkingLot::unparkOneImpl):
(WTF::ParkingLot::unparkAll):

• wtf/ParkingLot.h:

(WTF::ParkingLot::parkConditionally):
(WTF::ParkingLot::compareAndPark):
(WTF::ParkingLot::unparkOne):

Tools:

• TestWebKitAPI/Tests/WTF/ParkingLot.cpp:

Location:

trunk

Files:

• Source/WTF/ChangeLog (modified) (1 diff)
• Source/WTF/benchmarks/LockFairnessTest.cpp (modified) (5 diffs)
• Source/WTF/benchmarks/ToyLocks.h (modified) (2 diffs)
• Source/WTF/wtf/Condition.h (modified) (2 diffs)
• Source/WTF/wtf/Lock.cpp (modified) (5 diffs)
• Source/WTF/wtf/Lock.h (modified) (5 diffs)
• Source/WTF/wtf/ParkingLot.cpp (modified) (19 diffs)
• Source/WTF/wtf/ParkingLot.h (modified) (5 diffs)
• Tools/ChangeLog (modified) (1 diff)
• Tools/TestWebKitAPI/Tests/WTF/ParkingLot.cpp (modified) (1 diff)

trunk/Source/WTF/ChangeLog

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 2016-07-17  Myles C. Maxfield  <mmaxfield@apple.com>
 

trunk/Source/WTF/benchmarks/LockFairnessTest.cpp

@@ -49 +49 @@
 NO_RETURN void usage()
 {
-    printf("Usage: LockFairnessTest yieldspinlock|pausespinlock|wordlock|lock|barginglock|bargingwordlock|thunderlock|thunderwordlock|cascadelock|cascadewordlockhandofflock|mutex|all <num threads> <seconds per test>\n");
+    printf("Usage: LockFairnessTest yieldspinlock|pausespinlock|wordlock|lock|barginglock|bargingwordlock|thunderlock|thunderwordlock|cascadelock|cascadewordlockhandofflock|mutex|all <num threads> <seconds per test>\n");
     exit(1);
 }
@@ -55 +55 @@
 unsigned numThreads;
 double secondsPerTest;
+
 
 struct Benchmark {
@@ -73 +74 @@
                 "Benchmark Thread",
                 [&, threadIndex] () {
+
+
+
+
+
+
+
+
+
                     while (keepGoing) {
                         lock.lock();
                         counts[threadIndex]++;
+
                         lock.unlock();
                     }
@@ -86 +97 @@
         sleep(secondsPerTest);
     
+
         lock.lock();
-        keepGoing = false;
     
         dataLog(name, ": ");
@@ -107 +118 @@
     WTF::initializeThreading();
     
-    if (argc != 4
+    if (argc != 
         || sscanf(argv[2], "%u", &numThreads) != 1
-        || sscanf(argv[3], "%lf", &secondsPerTest) != 1)
+        || sscanf(argv[3], "%lf", &secondsPerTest) != 1
+        || sscanf(argv[4], "%u", &microsecondsInCriticalSection) != 1)
         usage();
     

trunk/Source/WTF/benchmarks/ToyLocks.h

@@ -236 +236 @@
         ParkingLot::unparkOne(
             &m_state,
-            [this] (ParkingLot::UnparkResult result) {
+            [this] (ParkingLot::UnparkResult result) {
                 if (result.mayHaveMoreThreads)
                     m_state.store(hasParkedBit);
                 else
                     m_state.store(0);
+
             });
     }
@@ -431 +432 @@
             
             if (m_state.compareExchangeWeak(state, state + parkedCountUnit)) {
-                bool result = ParkingLot::compareAndPark(&m_state, state + parkedCountUnit);
+                bool result = ParkingLot::compareAndPark(&m_state, state + parkedCountUnit);
                 m_state.exchangeAndAdd(-parkedCountUnit);
                 if (result)

trunk/Source/WTF/wtf/Condition.h

@@ -81 +81 @@
                 },
                 [&lock] () { lock.unlock(); },
-                timeout);
+                timeout);
         }
         lock.lock();
@@ -181 +181 @@
         ParkingLot::unparkOne(
             &m_hasWaiters,
-            [this] (ParkingLot::UnparkResult result) {
+            [this] (ParkingLot::UnparkResult result) {
                 if (!result.mayHaveMoreThreads)
                     m_hasWaiters.store(false);
+
             });
     }

trunk/Source/WTF/wtf/Lock.cpp

@@ -1 +1 @@
 /*
- * Copyright (C) 2015 Apple Inc. All rights reserved.
+ * Copyright (C) 2015 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -68 +68 @@
 
         // We now expect the value to be isHeld|hasParked. So long as that's the case, we can park.
-        ParkingLot::compareAndPark(&m_byte, isHeldBit | hasParkedBit);
+        ParkingLot::ParkResult parkResult =
+            ParkingLot::compareAndPark(&m_byte, isHeldBit | hasParkedBit);
+        if (parkResult.wasUnparked) {
+            switch (static_cast<Token>(parkResult.token)) {
+            case DirectHandoff:
+                // The lock was never released. It was handed to us directly by the thread that did
+                // unlock(). This means we're done!
+                RELEASE_ASSERT(isHeld());
+                return;
+            case BargingOpportunity:
+                // This is the common case. The thread that called unlock() has released the lock,
+                // and we have been woken up so that we may get an opportunity to grab the lock. But
+                // other threads may barge, so the best that we can do is loop around and try again.
+                break;
+            }
+        }
 
         // We have awoken, or we never parked because the byte value changed. Either way, we loop
@@ -75 +90 @@
 }
 
-NEVER_INLINE void LockBase::unlockSlow()
+void LockBase::unlockSlow()
+{
+    unlockSlowImpl(Unfair);
+}
+
+void LockBase::unlockFairlySlow()
+{
+    unlockSlowImpl(Fair);
+}
+
+NEVER_INLINE void LockBase::unlockSlowImpl(Fairness fairness)
 {
     // We could get here because the weak CAS in unlock() failed spuriously, or because there is
@@ -90 +115 @@
         }
 
-        // Someone is parked. Unpark exactly one thread, possibly leaving the parked bit set if
-        // there is a chance that there are still other threads parked.
+        // Someone is parked. Unpark exactly one thread. We may hand the lock to that thread
+        // directly, or we will unlock the lock at the same time as we unpark to allow for barging.
+        // When we unlock, we may leave the parked bit set if there is a chance that there are still
+        // other threads parked.
         ASSERT(oldByteValue == (isHeldBit | hasParkedBit));
         ParkingLot::unparkOne(
             &m_byte,
-            [this] (ParkingLot::UnparkResult result) {
+            [ {
                 // We are the only ones that can clear either the isHeldBit or the hasParkedBit,
                 // so we should still see both bits set right now.
                 ASSERT(m_byte.load() == (isHeldBit | hasParkedBit));
+
+
+
+
+
+
 
                 if (result.mayHaveMoreThreads)
@@ -104 +137 @@
                 else
                     m_byte.store(0);
+
             });
         return;

trunk/Source/WTF/wtf/Lock.h

@@ -1 +1 @@
 /*
- * Copyright (C) 2015 Apple Inc. All rights reserved.
+ * Copyright (C) 2015 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -41 +41 @@
 // competetive to a spinlock (uncontended locking is inlined and is just a CAS, microcontention is
 // handled by spinning and yielding), and a slow path that is competetive to std::mutex (if a lock
-// cannot be acquired in a short period of time, the thread is put to sleep until the lock is available
-// again). It uses less memory than a std::mutex.
+// cannot be acquired in a short period of time, the thread is put to sleep until the lock is
+// available again). It uses less memory than a std::mutex. This lock guarantees eventual stochastic
+// fairness, even in programs that relock the lock immediately after unlocking it. Except when there
+// are collisions between this lock and other locks in the ParkingLot, this lock will guarantee that
+// at worst one call to unlock() per millisecond will do a direct hand-off to the thread that is at
+// the head of the queue. When there are collisions, each collision increases the fair unlock delay
+// by one millisecond in the worst case.
 
 // This is a struct without a constructor or destructor so that it can be statically initialized.
@@ -74 +79 @@
     }
 
+
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+
+
     void unlock()
     {
@@ -82 +95 @@
 
         unlockSlow();
+
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     }
 
@@ -102 +130 @@
     WTF_EXPORT_PRIVATE void lockSlow();
     WTF_EXPORT_PRIVATE void unlockSlow();
+
+
+
+
+
+
+
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+
+
+
+
 
     // Method used for testing only.

trunk/Source/WTF/wtf/ParkingLot.cpp

@@ -27 +27 @@
 #include "ParkingLot.h"
 
+
 #include "DataLog.h"
 #include "HashFunctions.h"
@@ -33 +34 @@
 #include "ThreadingPrimitives.h"
 #include "Vector.h"
+
 #include "WordLock.h"
 #include <condition_variable>
@@ -59 +61 @@
     
     ThreadData* nextInQueue { nullptr };
+
+
 };
 
@@ -70 +74 @@
     WTF_MAKE_FAST_ALLOCATED;
 public:
+
+
+
+
+
     void enqueue(ThreadData* data)
     {
@@ -124 +133 @@
         ThreadData** currentPtr = &queueHead;
         ThreadData* previous = nullptr;
+
+
+
+
+
+
+
+
         while (shouldContinue) {
             ThreadData* current = *currentPtr;
@@ -130 +147 @@
             if (!current)
                 break;
-            DequeueResult result = functor(current);
+            DequeueResult result = functor(current);
             switch (result) {
             case DequeueResult::Ignore:
@@ -144 +161 @@
                 if (current == queueTail)
                     queueTail = previous;
+
                 *currentPtr = current->nextInQueue;
                 current->nextInQueue = nullptr;
@@ -151 +169 @@
             }
         }
+
+
+
 
         ASSERT(!!queueHead == !!queueTail);
@@ -159 +180 @@
         ThreadData* result = nullptr;
         genericDequeue(
-            [&] (ThreadData* element) -> DequeueResult {
+            [&] (ThreadData* element) -> DequeueResult {
                 result = element;
                 return DequeueResult::RemoveAndStop;
@@ -172 +193 @@
     // this lock.
     WordLock lock;
+
+
+
+
 
     // Put some distane between buckets in memory. This is one of several mitigations against false
@@ -531 +556 @@
 } // anonymous namespace
 
-NEVER_INLINE bool ParkingLot::parkConditionallyImpl(
+NEVER_INLINE  ParkingLot::parkConditionallyImpl(
     const void* address,
     const ScopedLambda<bool()>& validation,
@@ -541 +566 @@
     
     ThreadData* me = myThreadData();
+
 
     // Guard against someone calling parkConditionally() recursively from beforeSleep().
     RELEASE_ASSERT(!me->address);
 
-    bool result = enqueue(
+    bool esult = enqueue(
         address,
         [&] () -> ThreadData* {
@@ -555 +581 @@
         });
 
-    if (!result)
-        return false;
+    if (!esult)
+        return ;
 
     beforeSleep();
@@ -583 +609 @@
     if (didGetDequeued) {
         // Great! We actually got dequeued rather than the timeout expiring.
-        return true;
+        ParkResult result;
+        result.wasUnparked = true;
+        result.token = me->token;
+        return result;
     }
 
     // Have to remove ourselves from the queue since we timed out and nobody has dequeued us yet.
 
-    // It's possible that we get unparked right here, just before dequeue() grabs a lock. It's
-    // probably worthwhile to detect when this happens, and return true in that case, to ensure
-    // that when we return false it really means that no unpark could have been responsible for us
-    // waking up, and that if an unpark call did happen, it woke someone else up.
+    bool didDequeue = false;
     dequeue(
         address, BucketMode::IgnoreEmpty,
-        [&] (ThreadData* element) {
-            if (element == me)
+        [&] (ThreadData* element, bool) {
+            if (element == me) {
+                didDequeue = true;
                 return DequeueResult::RemoveAndStop;
+
             return DequeueResult::Ignore;
         },
         [] (bool) { });
-
-    ASSERT(!me->nextInQueue);
+    
+    // If didDequeue is true, then we dequeued ourselves. This means that we were not unparked.
+    // If didDequeue is false, then someone unparked us.
+    
+    RELEASE_ASSERT(!me->nextInQueue);
 
     // Make sure that no matter what, me->address is null after this point.
     {
         std::lock_guard<std::mutex> locker(me->parkingLock);
+
+
+
+
         me->address = nullptr;
     }
 
-    // If we were not found in the search above, then we know that someone unparked us.
-    return false;
+    ParkResult result;
+    result.wasUnparked = !didDequeue;
+    if (!didDequeue) {
+        // If we were unparked then there should be a token.
+        result.token = me->token;
+    }
+    return result;
 }
 
@@ -624 +664 @@
         address,
         BucketMode::EnsureNonEmpty,
-        [&] (ThreadData* element) {
+        [&] (ThreadData* element) {
             if (element->address != address)
                 return DequeueResult::Ignore;
@@ -644 +684 @@
         std::unique_lock<std::mutex> locker(threadData->parkingLock);
         threadData->address = nullptr;
+
     }
     threadData->parkingCondition.notify_one();
@@ -652 +693 @@
 NEVER_INLINE void ParkingLot::unparkOneImpl(
     const void* address,
-    const ScopedLambda<void(ParkingLot::UnparkResult)>& callback)
+    const ScopedLambda<(ParkingLot::UnparkResult)>& callback)
 {
     if (verbose)
         dataLog(toString(currentThread(), ": unparking one the hard way.\n"));
-
+    
     ThreadData* threadData = nullptr;
+
     dequeue(
         address,
         BucketMode::EnsureNonEmpty,
-        [&] (ThreadData* element) {
+        [&] (ThreadData* element) {
             if (element->address != address)
                 return DequeueResult::Ignore;
             threadData = element;
+
             return DequeueResult::RemoveAndStop;
         },
@@ -671 +714 @@
             result.didUnparkThread = !!threadData;
             result.mayHaveMoreThreads = result.didUnparkThread && mayHaveMoreThreads;
-            callback(result);
+            if (timeToBeFair)
+                RELEASE_ASSERT(threadData);
+            result.timeToBeFair = timeToBeFair;
+            intptr_t token = callback(result);
+            if (threadData)
+                threadData->token = token;
         });
 
@@ -695 +743 @@
         address,
         BucketMode::IgnoreEmpty,
-        [&] (ThreadData* element) {
+        [&] (ThreadData* element) {
             if (verbose)
                 dataLog(toString(currentThread(), ": Observing element with address = ", RawPointer(element->address), "\n"));

trunk/Source/WTF/wtf/ParkingLot.h

@@ -44 +44 @@
     // Parks the thread in a queue associated with the given address, which cannot be null. The
     // parking only succeeds if the validation function returns true while the queue lock is held.
+
     // If validation returns false, it will unlock the internal parking queue and then it will
-    // return without doing anything else. If validation returns true, it will enqueue the thread,
-    // unlock the parking queue lock, call the beforeSleep function, and then it will sleep so long
-    // as the thread continues to be on the queue and the timeout hasn't fired. Finally, this
-    // returns true if we actually got unparked or false if the timeout was hit. Note that
-    // beforeSleep is called with no locks held, so it's OK to do pretty much anything so long as
-    // you don't recursively call parkConditionally(). You can call unparkOne()/unparkAll() though.
-    // It's useful to use beforeSleep() to unlock some mutex in the implementation of
+    // return a null ParkResult (wasUnparked = false, token = 0) without doing anything else.
+    //
+    // If validation returns true, it will enqueue the thread, unlock the parking queue lock, call
+    // the beforeSleep function, and then it will sleep so long as the thread continues to be on the
+    // queue and the timeout hasn't fired. Finally, this returns wasUnparked = true if we actually
+    // got unparked or wasUnparked = false if the timeout was hit. When wasUnparked = true, the
+    // token will contain whatever token was returned from the callback to unparkOne(), or 0 if the
+    // thread was unparked using unparkAll() or the form of unparkOne() that doesn't take a
+    // callback.
+    //
+    // Note that beforeSleep is called with no locks held, so it's OK to do pretty much anything so
+    // long as you don't recursively call parkConditionally(). You can call unparkOne()/unparkAll()
+    // though. It's useful to use beforeSleep() to unlock some mutex in the implementation of
     // Condition::wait().
+
+
+
+
     template<typename ValidationFunctor, typename BeforeSleepFunctor>
-    static bool parkConditionally(
+    static  parkConditionally(
         const void* address,
         ValidationFunctor&& validation,
@@ -70 +81 @@
     // indefinitely so long as the value at the given address hasn't changed.
     template<typename T, typename U>
-    static bool compareAndPark(const Atomic<T>* address, U expected)
+    static  compareAndPark(const Atomic<T>* address, U expected)
     {
         return parkConditionally(
@@ -82 +93 @@
     }
 
+
+
+
+
+
+
+
+
+
+
+
+
     // Unparks one thread from the queue associated with the given address, which cannot be null.
     // Returns true if there may still be other threads on that queue, or false if there definitely
     // are no more threads on the queue.
-    struct UnparkResult {
-        bool didUnparkThread { false };
-        bool mayHaveMoreThreads { false };
-    };
     WTF_EXPORT_PRIVATE static UnparkResult unparkOne(const void* address);
 
+
+
+
     // Unparks one thread from the queue associated with the given address, and calls the given
-    // functor while the address is locked. Reports to the callback whether any thread got unparked
-    // and whether there may be any other threads still on the queue. This is an expert-mode version
-    // of unparkOne() that allows for really good thundering herd avoidance in adaptive mutexes.
-    // Without this, a lock implementation that uses unparkOne() has to have some trick for knowing
-    // if there are still threads parked on the queue, so that it can set some bit in its lock word
-    // to indicate that the next unlock() also needs to unparkOne(). But there is a race between
-    // manipulating that bit and some other thread acquiring the lock. It's possible to work around
-    // that race - see Rusty Russel's well-known usersem library - but it's not pretty. This form
-    // allows that race to be completely avoided, since there is no way that a thread can be parked
-    // while the callback is running.
+    // 
+    // 
+    // 
+    // 
+    //
+    // 
+    // 
+    // 
+    // 
+    // .
     template<typename Callback>
     static void unparkOne(const void* address, Callback&& callback)
     {
-        unparkOneImpl(address, scopedLambda<void(UnparkResult)>(std::forward<Callback>(callback)));
+        unparkOneImpl(address, scopedLambda<(UnparkResult)>(std::forward<Callback>(callback)));
     }
 
@@ -127 +149 @@
 
 private:
-    WTF_EXPORT_PRIVATE static bool parkConditionallyImpl(
+    WTF_EXPORT_PRIVATE static  parkConditionallyImpl(
         const void* address,
         const ScopedLambda<bool()>& validation,
@@ -134 +156 @@
     
     WTF_EXPORT_PRIVATE static void unparkOneImpl(
-        const void* address, const ScopedLambda<void(UnparkResult)>& callback);
+        const void* address, const ScopedLambda<(UnparkResult)>& callback);
 
     WTF_EXPORT_PRIVATE static void forEachImpl(const std::function<void(ThreadIdentifier, const void*)>&);

trunk/Tools/ChangeLog

@@ +1 @@
+
+
+
+
+
+
+
+
+
 2016-07-17  Sam Weinig  <sam@webkit.org>
 

trunk/Tools/TestWebKitAPI/Tests/WTF/ParkingLot.cpp

@@ -149 +149 @@
             
             // We need to wait.
-            if (ParkingLot::compareAndPark(&semaphore, newSemaphoreValue)) {
+            if (ParkingLot::compareAndPark(&semaphore, newSemaphoreValue)) {
                 // We did wait, and then got woken up. This means that someone who up'd the semaphore
                 // passed ownership onto us.