[PATCH] nptl: Fix racy pipe closing in tst-cancel{20,21}

[Paul E. Murphy]

This looks ok to me. Failing with a timeout seems better than
incorrectly reporting a pass. Though, a 40 second timeout seems
a bit high to me.

Tested on ppc64.

Paul

On 10/29/2015 12:28 PM, Adhemerval Zanella wrote:
> Ping.
> 
> On 19-10-2015 17:01, Adhemerval Zanella wrote:
>> Ping.
>>
>> On 14-10-2015 16:58, Rich Felker wrote:
>>> On Wed, Oct 14, 2015 at 04:03:15PM -0300, Adhemerval Zanella wrote:
>>>> The tst-cancel20 open two pipes and creates a thread which blocks
>>>> reading the first pipe.  It then issues a signal to activate an
>>>> handler which also blocks reading the second pipe.  Finally the
>>>> cancellation cleanup-up handlers are tested by first closing the
>>>> all the pipe ends and issuing a pthread_cancel. The tst-cancel21 
>>>> have a similar behavior, but use an extra fork after the test itself.
>>>>
>>>> The race condition occurs if the cancellation handling acts after the
>>>> pipe close: in this case read will return EOF (indicating side-effects)
>>>> and thus the cancellation must not act.  However current GLIBC
>>>> cancellation behavior acts regardless the syscalls returns with
>>>> sid-effects.
>>>>
>>>> This patch adjust the test by moving the pipe closing after the
>>>> cancellation handling.  This avoid spurious cancellation for the
>>>> case described.
>>>>
>>>> Checked on x86_64 and i386.
>>>
>>> I was involved in the discussion of this and believe that the fix is
>>> correct. The only reason the tests "worked" before was that
>>> cancellation was wrongly being acted upon after read succeeded in
>>> reading EOF.
>>>
>>> Note that, with this change, the tests will now timeout if read fails
>>> to act on cancellation, rather than exiting with a reportable error.
>>> This could be fixed with some very complicated machinery involving an
>>> additional signal handler and AS-safe synchronization mechanisms to
>>> control the ordering of close with respect to interruption of read,
>>> but as long as timeout is an acceptable way of detecting test failure,
>>> I see no reason to complicate the test logic like that.
>>>
>>> Rich
>>>
>