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 <[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 */
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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