[Intel-gfx] [PATCH] igt_core: zero exit_handler_count before forking

[Paulo Zanoni]

2014-09-04 5:48 GMT-03:00 Chris Wilson <chris at chris-wilson.co.uk>:
> On Thu, Sep 04, 2014 at 10:39:31AM +0200, Daniel Vetter wrote:
>> On Thu, Sep 04, 2014 at 10:34:13AM +0200, Daniel Vetter wrote:
>> > On Wed, Sep 03, 2014 at 02:47:21PM -0300, Paulo Zanoni wrote:
>> > > From: Paulo Zanoni <paulo.r.zanoni at intel.com>
>> > >
>> > > If we don't reset exit_handler_count before forking, we may have a
>> > > case where the forked process is killed before it even does
>> > > "exit_handler_count = 0": in that case, it is still finishing forking.
>> > > When that happens, we may end up calling our exit handlers. On the
>> > > specific bug I'm investigating, we call igt_reset_connnectors(), which
>> > > ends up in a deadlock inside malloc_atfork. If we attach gdb to the
>> > > forked process and get a backtrace, we have:
>> > >
>> > > (gdb) bt
>> > > 0  __lll_lock_wait_private () at ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
>> > > 1  0x00007f15634d36bf in _L_lock_10524 () from /lib/x86_64-linux-gnu/libc.so.6
>> > > 2  0x00007f15634d12ef in malloc_atfork (sz=139729840351352, caller=<optimized out>) at arena.c:181
>> > > 3  0x00007f15640466a1 in drmMalloc () from /usr/lib/x86_64-linux-gnu/libdrm.so.2
>> > > 4  0x00007f1564049ad7 in drmModeGetResources () from /usr/lib/x86_64-linux-gnu/libdrm.so.2
>> > > 5  0x0000000000408f84 in igt_reset_connectors () at igt_kms.c:1656
>> > > 6  0x00000000004092dc in call_exit_handlers (sig=15) at igt_core.c:1130
>> > > 7  fatal_sig_handler (sig=15) at igt_core.c:1154
>> > > 8  <signal handler called>
>> > > 9  0x00007f15634cce60 in ptmalloc_unlock_all2 () at arena.c:298
>> > > 10 0x00007f156350ca3f in __libc_fork () at ../nptl/sysdeps/unix/sysv/linux/x86_64/../fork.c:188
>> > > 11 0x000000000040a029 in __igt_fork_helper (proc=proc at entry=0x610fc4 <signal_helper>) at igt_core.c:910
>> > > 12 0x000000000040459d in igt_fork_signal_helper () at igt_aux.c:110
>> > > 13 0x0000000000402ab7 in __real_main63 () at bug.c:76
>> > > 14 0x000000000040296e in main (argc=<optimized out>, argv=<optimized out>) at bug.c:63
>
> But note that this means that igt_reset_connectors is *not* signal safe
> and needs to be rewritten in order for it to work as an exit handler.
> Otherwise this bug will just keep on reoccurring.

Yeah, I studied the problem a little bit more and now I agree. I'm
hoping the current patch will at least unblock QA's testing efforts,
giving more time for us to write the proper solution. I'll keep
watching the original bug report. Here is the bug report for the
"real"fix: https://bugs.freedesktop.org/show_bug.cgi?id=83498 .

> -Chris
>
> --
> Chris Wilson, Intel Open Source Technology Centre



-- 
Paulo Zanoni