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From: "andi-bz at firstfloor dot org" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug nptl/15771] New: __lll_unlock_wake break perf/libunwind unwinding
Date: Tue, 23 Jul 2013 04:16:00 -0000
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http://sourceware.org/bugzilla/show_bug.cgi?id=15771

            Bug ID: 15771
           Summary: __lll_unlock_wake break perf/libunwind unwinding
           Product: glibc
           Version: 2.17
            Status: NEW
          Severity: normal
          Priority: P2
         Component: nptl
          Assignee: unassigned at sourceware dot org
          Reporter: andi-bz at firstfloor dot org
                CC: drepper.fsp at gmail dot com

Created attachment 7113
  --> http://sourceware.org/bugzilla/attachment.cgi?id=7113&action=edit
mutex test case

Something in the way the x86-64 dwarf unwind table is written for
__lll_unlock_wake breaks perf's libunwind implementation

This is a problem because it makes it hard to track down lock contention in
pthread programs.

When profiling the attached test case with 

perf record -g dwarf ...
perf report 

we get

perf/libunwind:
 |--57.28%-- futex_wake
                 |          do_futex
                 |          sys_futex
                 |          system_call_fastpath
                 |          __lll_unlock_wake
                 |          |         
                 |          |--80.89%-- 0x0
                 |          |         
                 |          |--18.73%-- _L_unlock_701
                 |           --0.38%-- [...]


The unwind always stops inside __lll_unlock_wake

gdb can backtrace through it

#0  0x000000326fe0de8a in __lll_unlock_wake () from /lib64/libpthread.so.0
#1  0x000000326fe0aae3 in _L_unlock_701 () from /lib64/libpthread.so.0
#2  0x000000326fe0aa6e in pthread_mutex_unlock () from /lib64/libpthread.so.0
#3  0x0000000000400809 in thread (arg=0x0) at tmutex.c:14
#4  0x000000326fe07c53 in start_thread () from /lib64/libpthread.so.0
#5  0x000000326faf513d in clone () from /lib64/libc.so.6

The glibc backtrace used by mutrace also seems to work.

This is on FC19 with 3.10 perf and libunwind-1.1-2.fc19.x86_64

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