Async cacellation and pthread_cleanup_push

[Andreas Schwab]

On Aug 23 2021, Florian Weimer wrote:

> * Andreas Schwab:
>
>> On Aug 04 2021, Florian Weimer wrote:
>>
>>> * Andreas Schwab:
>>>
>>>> On Aug 04 2021, Florian Weimer wrote:
>>>>
>>>>> * Andreas Schwab:
>>>>>
>>>>>> Is pthread_cleanup_push supposed to work together with async
>>>>>> cancellation?
>>>>>
>>>>> In our implementation?  I think so.
>>>>
>>>> I don't see how that can work with -fexceptions or in C++, though.  Is
>>>> that supported?
>>>
>>> I think it is supposed to work, but it probably is unreliable.
>>
>> Since the compiler puts the exception regions only over function calls
>> (and -fnon-call-exceptions only covers potentially trapping insns in
>> addition) the cleanup region will not be fully covered.
>
> I can see how this can happen in theory …

This is not theory, it happens exactly here.

>> testcases/open_posix_testsuite/conformance/interfaces/pthread_cancel/3-1.c
>
> … but I don't see how this applies to this particular test.

There is a non-zero probability that the cancel signal arrives in
thread_func and sleep_loop while it is executing the unprotected
instructions outside of the .LEHB1 to .LEHE1 region:

        movl    $sem, %edi
        call    sem_post
        movq    $0, (%rsp)
        movq    $1000000, 8(%rsp)
        .p2align 4,,10
        .p2align 3
.L2:
        xorl    %esi, %esi
        movq    %rsp, %rdi
.LEHB1:
        call    nanosleep
        subl    $1, %ebx
        jne     .L2
        movl    $.LC0, %edi
        call    puts
.LEHE1:

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
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