Jarek Poplawski wrote, On 11/28/2007 11:56 PM:
> 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 */
...But, not for sure! If our caller holds the lock and we can
check this...
>> ...
>> /* 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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