[Bug nptl/6955] New: PRIO_INHERIT mutexes do not use FUTEX_PRIVATE_FLAG

["ben at ben dot com" <sourceware-bugzilla at sourceware dot org>]

Code in the PRIO_INHERIT cases of pthread_mutex_lock/unlock/trylock/timedlock
call futex() directly with FUTEX_(UN)LOCK_PI regardless of the pshared-ness of
the mutex.  Other cases call futex() indirectly through __lll_*() interfaces
that set FUTEX_PRIVATE_FLAG as appropriate.  This was observed to cause
contention on mm->mmap_sema leading to large, random latencies on otherwise
quick lock operations (apparently due to brk() holding mmap_sema).

I patched nptl to set the private flag in the PRIO_INHERIT cases (based on the
pshared-ness of the mutex) and tested that against the regression tests and my
application.  The tests pass, the application works, and the performance problem
is solved.  However, I am working from a fairly superficial understanding of
this code, and I have not received replies from my inquiries to experts in this
code.

I will try to attach my patch after I submit.  I think it illustrates the
desired behavior, but it may not be exactly what glibc maintainers would want.

-- 
           Summary: PRIO_INHERIT mutexes do not use FUTEX_PRIVATE_FLAG
           Product: glibc
           Version: 2.8
            Status: NEW
          Severity: normal
          Priority: P2
         Component: nptl
        AssignedTo: drepper at redhat dot com
        ReportedBy: ben at ben dot com
                CC: glibc-bugs at sources dot redhat dot com
 GCC build triplet: x86_64-redhat-linux-gnu
  GCC host triplet: x86_64-redhat-linux-gnu
GCC target triplet: x86_64-linux


http://sourceware.org/bugzilla/show_bug.cgi?id=6955

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