On Wed, 28 Nov 2007 23:45:13 +0100
Jarek Poplawski <[EMAIL PROTECTED]> wrote:
> Larry Finger wrote, On 11/28/2007 04:41 PM:
>
> > Andreas Schwab wrote:
> >> Larry Finger <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]>
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