Re: [PATCH] pthread_once hangs when init routine throws an exception [BZ #18435]

["Carlos O'Donell" <carlos at redhat dot com>]

On 07/08/2015 12:52 PM, Szabolcs Nagy wrote:
> On 08/07/15 17:33, Torvald Riegel wrote:
>> On Wed, 2015-07-08 at 12:09 -0400, Carlos O'Donell wrote:
>>> On 07/08/2015 07:00 AM, Szabolcs Nagy wrote:
>>>> (2) Should gcc support exceptions from async signal handlers?
>>>
>>> No. I don't think you can support it safely.
>>>
>>>> nptl/tst-join5 failure is more problematic: it fails because gcc
>>>> does not seem to implement -fexceptions with the assumption that
>>>> signal handlers can throw, in particular it assumes inline asm
>>>> does not throw exceptions.  If the syscall that is a cancellation
>>>> point appears between pthread_cleanup_push and pthread_cleanup_pop
>>>> in glibc internal code, the cleanup handler may not get run on
>>>> cancellation depending on where gcc moved the syscall inline asm.
>>>> (It is free to move it outside the code range that is marked for
>>>> exception handling, this is what happens on aarch64 in pthread_join).
>>>> This affects all archs, but some may get lucky.
>>>
>>> Ah! That's truly a terrible scenario.
>>>
>>>> (My understanding: gcc must be very strict about how it marks
>>>> the code range for exception handling and assume any instruction
>>>> may throw if it wants -fexceptions -fasynchronous-unwind-tables to
>>>> work from signal handlers.  Current compilers do not seem to support
>>>> this so glibc internal code should not rely on it, which means the
>>>> cancellation mechanism should not rely on exception handling at
>>>> least not when the exception is thrown from the cancel signal
>>>> handler.  I think the gnu toolchain should not try to make pthread
>>>> cancellation to interoperate with C++ exceptions nor to make
>>>> exceptions work from signal handlers: no standard requires this
>>>> behaviour and seems to cause problems).
>>>
>>> No, we just need to revert this patch and have C++ implement
>>> std::call_once by itself.
>>
>> Would point (2) be taken care of by Adhemerval's cancellation changes?
>>
> 
> yes: if the cancel point syscall is not inline asm,
> but extern call (that is not marked with nothrow)
> then gcc -fexceptions should handle it correctly.
> 
> asynchronous cancellation is still problematic,
> but that is a special case.

And we still have to support that case which makes this change
a net loss of functionality. Therefore I think we need to revert
this and try again 2.23.

Cheers,
Carlos.