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

On 03/18/2010 10:51 AM, Daniel P. Berrange wrote: > On Thu, Mar 18, 2010 at 10:41:58AM -0600, Eric Blake wrote: >> On 03/18/2010 09:15 AM, Stefan Berger wrote: >>> This patch adds recursive locks necessary due to the processing of >>> network filter XML that can reference other network filters, including >>> references that cause looks. Loops in the XML are prevented but their >>> detection requires recursive locks. >> >> ACK. I had to double-check MSDN to make sure that threads-win32 already >> creates recursive mutex by default. > > I don't supposed you read if its possible to make it non-recursive, since > the orginal impl is supposed to be non-recursive ! 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.