Re: [libvirt] [PATCH 1/14] Adding recursive locks

On 03/18/2010 10:51 AM, Daniel P. Berrange wrote: Unfortunately, I don't see a way: http://msdn.microsoft.com/en-us/library/ms684266%28VS.85%29.aspx states: After a thread obtains ownership of a mutex, it can specify the same mutex in repeated calls to the wait-functions without blocking its execution. This prevents a thread from deadlocking itself while waiting for a mutex that it already owns. To release its ownership under such circumstances, the thread must call ReleaseMutex once for each time that the mutex satisfied the conditions of a wait function. with no mention of any way to parameterize it to be non-recursive. But what does a true non-recursive mutex buy you? The only difference between recursive and true non-recursive is whether you declare that an attempt to relock a mutex that you already own is a fatal deadlock error, rather than incrementing a counter for matching unlocks. It's just that non-recursive mutexes typically have faster implementations. For that matter, do we even need the distinction? Maybe ALL our code should be using recursive mutexes by default, by changing virMutexInit to be recursive no matter what, and not worry about introducing virMutexInitRecursive. Looking more closely at virMutexInit in the pthreads version, we use pthread_mutex_init(,NULL), which requests PTHREAD_MUTEX_DEFAULT. But POSIX states: http://www.opengroup.org/onlinepubs/9699919799/functions/pthread_mutexatt... PTHREAD_MUTEX_DEFAULT Attempting to recursively lock a mutex of this type results in undefined behavior. Attempting to unlock a mutex of this type which was not locked by the calling thread results in undefined behavior. Attempting to unlock a mutex of this type which is not locked results in undefined behavior. An implementation may map this mutex to one of the other mutex types. That is, our current implementation of virMutexInit is NOT a true non-recursive mutex, so much as a mutex that is unspecified whether it is recursive or not. -- Eric Blake eblake(a)redhat.com +1-801-349-2682 Libvirt virtualization library http://libvirt.org