Re: WaitForSingleObject() will not deadlock

"Alexander Grigoriev" <>
Tue, 3 Jul 2007 09:13:59 -0700
A synchronization primitive without memory visibility guarantee CANNOT be
used to synchronize access to some data set, thus it's pretty much useless.
You might as well not use it at all, with the same result.

Atomic functions (at least on x86 and x64), DO provide memory fence.
CRITICAL_SECTION simply uses InterlockedCompareExchange. I think
pthread_mutex is no different.

Having said that, implementation of reentrant intra-thread mutex primitive
is VERY trivial. You can cook such a thing in 10 minutes:

    LONG OwningThreadID;
    LONG ReentranceCount;
    LONG ContentionCount;
    HANDLE hEvent;

void MyEnterCriticalSection(MY_CRITICAL_SECTION * p)
    LONG CurrentThreadID = GetCurrentThreadId();
    while (1)
        InterlockedIncrement( & p->ContentionCount);
        LONG OwnerID = InterlockedCompareExchangeAcquire( &
p->OwningThreadID, CurrentThreadID, 0);
        if (CurrentThreadID == OwnerID)
            // recursive entrance
            InterlockedDecrement( & p->ContentionCount);
        if (OwnerID == 0)
            // was not owned. We own it now
            InterlockedDecrement( & p->ContentionCount);
        KeWaitForSingleObject(p->hEvent, INFINITE);

void MyLeaveCriticalSection(MY_CRITICAL_SECTION * p)
    ASSERT(GetCurrentThreadId() == p->OwningThreadID);


    if (p->ReentranceCount== 0)
        // we don't own it anymore
        p->OwningThreadID= 0;
        if (0 != InterlockedExchange(p->ContentionCount, 0))

Note that it DOES privide memory visibility.

"Frank Cusack" <> wrote in message

On Sun, 1 Jul 2007 21:35:36 -0700 "Alexander Grigoriev"
<> wrote:

CRITICAL_SECTION (EnterCriticalSection, LeaveCriticalSection) provides a
memory fence, too.

Seems expensive then. A pthreads semaphore (typically used to guard a
critical section) has no memory visibility guarantees and as such can
be implemented with so-called atomic ops instead of memory barriers.

CRITICAL_SECTION is fast, intra-process, recursive mutual exclusion
synchronization object.

Not as fast as a pthreads semaphore, since the CRITICAL_SECTION has
to execute a memory barrier (fence).

Kernel mutex (CreateMutex) can be used to synchronize _between_
too, but since it requires a roundtrip to kernel mode, its overhead is
than of CRITICAL_SECTION. A kernel mutex also takes care of priority
inversion, which CRITICAL_SECTION does not.

posix mutexes (good implementations, anyway) only require entry into
the kernel when they are contested. Uncontested mutexes are extremely
fast. Solaris has an "adaptive mutex" which only enters the kernel
after spinning for a bit first, which is consistent with the design
philosophy that you should only hold on to mutexes for very short
periods of time. (There are more rules but it's not important to my

It almost doesn't make sense that CRITICAL_SECTIONs execute a membar
since Windows really only runs on x86, which is TSO. This is probably
just a nod towards more widespread use (ie, less skilled programmers)
who might otherwise get the semantics wrong.


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