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Date: Sun, 22 Nov 2020 23:05:54 +0300
From: Арсений <a@...r0n.science>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com,
jwakely@...hat.com,
Florian Weimer <fw@...eb.enyo.de>
Subject: Re[2]: Mutexes are not unlocking
Hello,
I fixed the problem by making a workaround. Specifying -D_GLIBCXX_GTHREAD_USE_WEAK=0 forces libstdc++ headers do not use weak symbols. Mutexes are correctly locked and unlocked now.
>Воскресенье, 22 ноября 2020, 22:28 +03:00 от Rich Felker <dalias@...c.org>:
>
>On Sun, Nov 22, 2020 at 08:23:23PM +0100, Florian Weimer wrote:
>> * Rich Felker:
>>
>> > On Sun, Nov 22, 2020 at 09:43:35PM +0300, Арсений wrote:
>> >>
>> >> Hello,
>> >> The problem is that mutex is not got unlocked after the first unlock().
>> >>
>> >> libstdc++ uses a wrapper for pthread called gthreads. This wrapper
>> >> checks for the state of the mutex system. For
>> >> example, pthread_mutex_unlock() is called in a following way:
>> >>
>> >> static inline int
>> >> __gthread_mutex_unlock (__gthread_mutex_t *__mutex)
>> >> {
>> >> if (__gthread_active_p ())
>> >> return __gthrw_(pthread_mutex_unlock) (__mutex);
>> >> else
>> >> return 0;
>> >> }
>> >
>> > Yes. This code is invalid (it misinterprets weak symbol information to
>> > draw incorrect conclusions about whether threads may be in use) and
>> > thus is disabled in builds of gcc/libstdc++ targeting musl-based
>> > systems. GCC and glibc-based distro folks mostly don't care because
>> > it only breaks static linking, but some of them actually hack gcc's
>> > libpthread.a into one giant .o file to work around the problem rather
>> > than fixing this in gcc...
>>
>> GCC 11 has a fix (if used along with glibc 2.32), but I wonder if it's
>> going to run into a similar issue because inlined code from older GCC
>> versions uses a diverging version of the check.
>>
>> Jonathan, more context is here:
>>
>> < https://www.openwall.com/lists/musl/2020/11/22/1 >
>Uhg, that's a really nasty problem. Is __gthread_active_p() also
>inlined? Or can it be given a definition to mitigate the problem?
>
>Rich
——
Арсений
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