Re: pthread_mutex_unlock (was Re: sched_yield() makes OpenLDAP slow)

["Christopher Friesen" <cfriesen@nortel.com>]

Howard Chu wrote:

> But why does A take the mutex in the first place? Presumably because it 
> is about to execute a critical section. And also presumably, A will not 
> release the mutex until it no longer has anything critical to do; 
> certainly it could hold it longer if it needed to.

Suppose A is pulling job requests off a queue.

A takes the mutex because it is going to modify data protected by the 
mutex.  It then gives up the mutex when it's done modifying the data.

> If A still needed the mutex, why release it and reacquire it, why not 
> just hold onto it? The fact that it is being released is significant.

Suppose A then pulls another job request off the queue.  It just so 
happens that this job requires touching some data protected by the same 
mutex.  It would need to take the mutex again.

A doesn't necessarily know what data the various jobs will require it to 
access, so it doesn't know a priori what mutexes will be required.

Chris