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

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

[fweimer at redhat dot com]

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

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-

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

[ben at ben dot com]

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.

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

[ben at ben dot com]

------- Additional Comments From ben at ben dot com  2008-10-09 19:20 -------
Created an attachment (id=2988)
 --> (http://sourceware.org/bugzilla/attachment.cgi?id=2988&action=view)
Add FUTEX_PRIVATE_FLAG support for PRIO_INHERIT mutexes

as described in the original bug report

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

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

[drepper at redhat dot com]

------- Additional Comments From drepper at redhat dot com  2008-11-01 19:56 -------
I've checked in a slightly different, more efficient patch.

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           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


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

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