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Re: [PATCH] pthread_once hangs when init routine throws an exception [BZ #18435]
- From: Szabolcs Nagy <szabolcs dot nagy at arm dot com>
- To: Torvald Riegel <triegel at redhat dot com>, Carlos O'Donell <carlos at redhat dot com>
- Cc: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>, "libc-alpha at sourceware dot org" <libc-alpha at sourceware dot org>, Marcus Shawcroft <Marcus dot Shawcroft at arm dot com>
- Date: Wed, 08 Jul 2015 17:52:09 +0100
- Subject: Re: [PATCH] pthread_once hangs when init routine throws an exception [BZ #18435]
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- References: <556B7F10 dot 40209 at redhat dot com> <557741C5 dot 5060203 at redhat dot com> <559A8029 dot 1000705 at arm dot com> <559A8DAE dot 9040604 at gmail dot com> <559A9789 dot 3090805 at linaro dot org> <559AADC8 dot 4030409 at arm dot com> <559AB627 dot 2050006 at arm dot com> <559D02E2 dot 5000303 at arm dot com> <559D4B4D dot 7090703 at redhat dot com> <1436373181 dot 22407 dot 38 dot camel at localhost dot localdomain>
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.