Rakesh Agarwal Rakesh Agarwal - 3 years ago 291
C++ Question

Is there a limit on the number of threads that can simultaneously acquire reader lock (SRW)?

In my application, for the concurrency, I am using Slim Reader Writer lock on Windows and pthread_rwlock_t on Mac/Linux.

I am seeing a weird test failure that makes me wonder if there is a limit on the number of threads that can possess the reader right at a given time ?

Please answer this for both SRW locks and pthread_rwlock_t . Thanks !


The test creates 16 threads initialized to call the same proc, let say foo(). This intermittently hangs.

void foo(int id) //id is the thread ID
/* Acquire shared mutex ... */
AcquireReadLock(g_mutex); // calls AcquireSRWLockShared on windows
AtomicDecrement(&g_TotalNumberOfThreads); // calls InterLockedDecrement()
while (g_TotalNumberOfThreads != 0)
; //spin

Answer Source

The problem turned out to be that the variable g_TotalNumberOfThreads (in the above code in the question) was being optimized by the compiler for not being read all the time.

Marking the variable g_TotalNumberOfThreads as volatile fixed the problem. Thanks !

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