Re: [PATCH] nptl: Remove cancellation checks from sem_{timed}wait (BZ #23006)

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

On 03/07/2019 15:28, Florian Weimer wrote:
> * Adhemerval Zanella:
> 
>> On 25/06/2019 11:29, Adhemerval Zanella wrote:
>>>
>>>
>>> On 25/06/2019 10:36, Florian Weimer wrote:
>>>> * Adhemerval Zanella:
>>>>
>>>>> In any case, do you think we should make sem_{timed}wait not a cancellation
>>>>> entrypoint for 2.30? I can rework the patch if it is the case.
>>>>
>>>> Do we have any other form of blocking synchronization that is
>>>> cancellable?  If not, a cancellable semaphore wait operation would make
>>>> sense.  But then we should perform the cancellation check on the fast
>>>> path, too.
>>>
>>> pthread_cond_wait, pthread_cond_timedwait, and pthread_join as well the
>>> gnu extension pthread_timedjoin_np and pthread_cond_clockwait (not yet
>>> upstream). Depending of you definition of synchronization, you can also
>>> include sigtimedwait, sigwait, sigwaitinfo, wait, waitid, and waitpid.
>>>
>>> We explicit does not support cancellation for pthread_rwlock_rdlock,
>>> pthread_rwlock_timedrdlock, pthread_rwlock_timedwrlock, and
>>> pthread_rwlock_wrlock. I tend to see that pthread_rwlock_* are similar
>>> to pthread_mutex and the rationale layout on 'Thread Cancellation 
>>> Overview' also applies.
>>>
>>> So at least we have synchronization functions that explicit does not
>>> act for cancellation, we might extend it to sem_{timed}wait now that
>>> they are also listed on 'shall'.
>>>
>>> However, I still think this does not really follow along with the
>>> rationale exposed on the very issue that triggered it [1], which aimed
>>> imho to just remove the requirement of check for cancellation on fast
>>> path only. 
>>>
>>> [1] http://austingroupbugs.net/view.php?id=1076#c3938
>>>
>>
>> Florian, how should we proceed regarding it? Should we just move
>> sem_{timed}wait out of the cancellable entrypoints or act uppon just
>> for the potentially blocked case? For latter do you still think we 
>> should keep a compat symbol for early check?
> 
> Ugh, to be honest, I don't think any code change is required here.

By 'here' do you mean the patch or current implementation? I still think
we can use the Austin clarification to optimize the sem_{timed}wait
and remove the early checks for the non-blocked case. And I also don't
think it is worth to add a compatibility check for this specific
semantic.