Messages in this thread | | | Date | Wed, 28 Nov 2007 23:56:07 +0100 | From | Jarek Poplawski <> | Subject | Re: Question regarding mutex locking |
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Jarek Poplawski wrote, On 11/28/2007 11:45 PM:
> Larry Finger wrote, On 11/28/2007 04:41 PM: > >> Andreas Schwab wrote: >>> Larry Finger <Larry.Finger@lwfinger.net> writes: >>> >>>> If a particular routine needs to lock a mutex, but it may be entered with that mutex already locked, >>>> would the following code be SMP safe? >>>> >>>> hold_lock = mutex_trylock() >>>> >>>> ... >>>> >>>> if (hold_lock) >>>> mutex_unlock() >>> When two CPUs may enter the critical region at the same time, what is >>> the point of the mutex? Also, the first CPU may unlock the mutex while >>> the second one is still inside the critical region. >> Thank you for that answer. I think that I'm finally beginning to understand. > > Probably it would be faster without these "...", which look like > no man's land... > > hold_lock = mutex_trylock() > if (hold_lock) { > /* SMP safe */ > ... > mutex_unlock() > } else { > /* SMP unsafe */ > ... > /* maybe try again after some break or check */
OOPS! Of course, since it can be called with this lock held, any break is not enough: we can only check if there is a possibility that another thread is holding the lock.
> } > > Regards, > Jarek P.
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