Re: [PATCH 07/11] nptl: Remove CANCELING_BITMASK

[Florian Weimer <fweimer@redhat.com>]

* 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.

Thanks,
Florian