[Intel-gfx] [PATCH] igt_core: zero exit_handler_count before forking
Daniel Vetter
daniel at ffwll.ch
Thu Sep 4 10:34:13 CEST 2014
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
>
> After doing some searches for "stuck at malloc_atfork", it seems to me
> we probably shouldn't be doing any malloc calls at this point of the
> code, so the best way to do that is to make sure we can't really run
> the exit handlers.
>
> So on this patch, instead of resetting the exit handlers after
> forking, we reset them before forking, and then restore the original
> value on the parent process.
>
> I can reproduce this problem by running "./kms_flip --run-subtest
> 2x-flip-vs-modeset" under an infinite loop. Usually after a few
> hundred calls, we end up stuck on the deadlock mentioned above. QA
> says this problem happens every time, but I'm not sure what is the
> difference between our environments that makes the race condition so
> much easier for them.
>
> The kms_flip.c problem can be considered a regression introduced by:
> commit eef768f283466b6d7cb3f08381f72ccf3951dc99
> Author: Thomas Wood <thomas.wood at intel.com>
> Date: Wed Jun 18 14:28:43 2014 +0100
> tests: enable extra connectors in kms_flip and kms_pipe_crc_basic
>
> even though this commit is not the one that introduced the real
> problem.
>
> It is also possible to reproduce this problem with a few modifications
> to template.c:
> - Add a call to igt_enable_connectors() inside the first fixture.
> - Add igt_fork_signal_helper() and igt_stop_signal_helper() calls
> around subtest B.
>
> Cc: Thomas Wood <thomas.wood at intel.com>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=81367
> Signed-off-by: Paulo Zanoni <paulo.r.zanoni at intel.com>
Awesome piece of debugging!
But imo the commit message is missing the explanation why the forked child
exists so quickly - the reason is that for skipped tests the parent kills
it right away, before libc has completed the setup code in the child.
We've had fun with this earlier, see
commit 139c72f38a07c545f5a9ab5fa3750779987b9275
Author: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Date: Tue Dec 3 16:44:55 2013 +0000
drmtest: Avoid wrong PID/TID after clone races
and the earlier attempt at
commit 9298dfabd9658315df34616b1e9a10b3579a92bd
Author: Daniel Vetter <daniel.vetter at ffwll.ch>
Date: Fri Sep 13 16:38:59 2013 +0200
lib: add test for igt_fork_signal_helper
I've added a note and commment and merged the patch, thanks.
-Daniel
> ---
> lib/igt_core.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/lib/igt_core.c b/lib/igt_core.c
> index 7f14b17..e9a27e3 100644
> --- a/lib/igt_core.c
> +++ b/lib/igt_core.c
> @@ -895,6 +895,7 @@ bool __igt_fork_helper(struct igt_helper_process *proc)
> {
> pid_t pid;
> int id;
> + int tmp_count;
>
> assert(!proc->running);
> assert(helper_process_count < ARRAY_SIZE(helper_process_pids));
> @@ -904,16 +905,19 @@ bool __igt_fork_helper(struct igt_helper_process *proc)
>
> igt_install_exit_handler(fork_helper_exit_handler);
>
> + tmp_count = exit_handler_count;
> + exit_handler_count = 0;
> switch (pid = fork()) {
> case -1:
> + exit_handler_count = tmp_count;
> igt_assert(0);
> case 0:
> - exit_handler_count = 0;
> reset_helper_process_list();
> oom_adjust_for_doom();
>
> return true;
> default:
> + exit_handler_count = tmp_count;
> proc->running = true;
> proc->pid = pid;
> proc->id = id;
> --
> 2.1.0
>
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--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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