Messages in this thread | | | From | Stephen Hemminger <> | Subject | Re: Question regarding mutex locking | Date | Wed, 28 Nov 2007 15:33:12 -0800 |
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On Wed, 28 Nov 2007 23:45:13 +0100 Jarek Poplawski <jarkao2@o2.pl> wrote:
> 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 */ > } > > Regards, > Jarek P.
WTF are you teaching a lesson on how NOT to do locking?
Any code which has this kind of convoluted dependency on conditional locking is fundamentally broken.
-- Stephen Hemminger <shemminger@linux-foundation.org>
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