Hello,
 
Yes, it is inlined:
  0x00007ffff7e0e3e9 <+88>:    mov    0x3b98(%rip),%r12        # 0x7ffff7e11f88 
  0x00007ffff7e0e3f0 <+95>:    test   %r12,%r12 
  0x00007ffff7e0e3f3 <+98>:    je     0x7ffff7e0e3fd <testMutexPP(std::mutex&, char const*)+108> 
  0x00007ffff7e0e3f5 <+100>:   mov    %rbp,%rdi 
  0x00007ffff7e0e3f8 <+103>:   callq  0x7ffff7e0d270 <pthread_mutex_unlock@plt>
Weak reference to pthread_key_create() is stored at 0x7ffff7e11f88. On musl, this memory location contains 0x0. 
>Воскресенье, 22 ноября 2020, 22:28 +03:00 от Rich Felker <dalias@libc.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 
 
 
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Арсений