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

[Szabolcs Nagy <szabolcs dot nagy at arm dot com>]

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.