[PATCH 07/11] nptl: Remove CANCELING_BITMASK

[Adhemerval Zanella]

On 15/06/2021 20:33, Adhemerval Zanella wrote:
> 
> 
> On 15/06/2021 19:07, Florian Weimer wrote:
>> * Adhemerval Zanella via Libc-alpha:
>>
>>> The CANCELING_BITMASK is used as an optimization to avoid sending
>>> the signal when pthread_cancel is called in a concurrent manner.
>>>
>>> This requires then to put both the cancellation state and type on
>>> a shared state (cancelhandling), since 'pthread_cancel' checks
>>> whether cancellation is enabled and asynchrnous to either cancel
>>> itself of sending the signal.
>>>
>>> It also requires handle the CANCELING_BITMASK on
>>> __pthread_disable_asynccancel, however this is incurs in the same
>>> issues described on BZ#12683: the cancellation is acting even *after*
>>> the syscalls returns with user visible side-effects.
>>>
>>> This patch removes this optimization and simplifies the pthread
>>> cancellation implementation: pthread_cancel now first check if
>>> cancellation is already pending and if not always send a signal
>>> if the target is not itself.  The SIGCANCEL handler is also simpified
>>> since there is not need to setup a CAS loop.
>>>
>>> It also alows to move both the cancellation state and mode out of
>>> 'cancelhadling' (it is done in subsequent patches).
>>
>> It looks like this causes sporadic failures in nptl/tst-sem16 because
>> sem_wait fails with EINTR, revealing that cancellation is implemented
>> using a signal (something that must not be visible according to POSIX).
>>
>> This was not a problem before because pthread_cancel sent the signal
>> only when asynchronous cancellation was enabled.
>>
>> We either have to revert this, or push forward with the transition so
>> that we can install the SIGCANCEL handler with SA_RESTART.
> 
> Indeed now that SIGCANCEL is always sent by pthread_cancel we must
> install the signal handle with SA_RESTART, the signal handle will
> either act on cancellation or it should be not visible by the 
> application.
> 
> I don't think there is a need to revert this change, running nptl/tst-sem16
> with SA_RESTART I don't see the regression anymore with multiple thousands
> runs.
> 

I will send a patch to use SA_RESTART on cancellation.