Re: [PATCH 4/5] nptl: Fix sem_wait and sem_timedwait cancellation

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

On 09/09/2016 13:32, Torvald Riegel wrote:
> On Mon, 2016-09-05 at 20:07 +0200, Torvald Riegel wrote:
>> On Mon, 2016-08-22 at 11:27 -0300, Adhemerval Zanella wrote:
>>> This patch fixes both sem_wait and sem_timedwait cancellation point for
>>> uncontended case.  In this scenario only atomics are involved and thus
>>> the futex cancellable call is not issue and a pending cancellation signal
>>> is not handled.
>>
>> I have added a comment on the BZ explaining why I think this is NOTABUG.
> 
> I've looked at the POSIX rationale again, and it seems I was wrong in my
> interpretation of what the POSIX spec requires.  I now think that we
> need to add the pthread_testcancel call or equivalent.  The patch looks
> okay, but please also add a comment to the added pthread_testcancel call
> explaining why we need it.  Something like this may be useful:
> 
> We need to check whether we need to act upon a cancellation request here
> because POSIX specifies that cancellation points "shall occur" in
> sem_wait and sem_timedwait, which also means that they need to check
> this regardless whether they block or not (unlike "may occur"
> functions).  See the POSIX Rationale for this requirement: Section
> "Thread Cancellation Overview" in
> http://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xsh_chap02.html
> See http://austingroupbugs.net/view.php?id=1076 for thoughts on why this
> may be a suboptimal design.
> 

Thanks for check out this, my initial understanding followed your #4 comment
in bug report.  I will add your comment in the patch.

> 
> Thanks.
>