linux-kernel - Re: [PATCH 5/5] coredump: abort core dump piping only due to a fatal signal

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Date:	Fri, 15 Feb 2013 17:20:04 -0800
From:	Mandeep Singh Baines <msb@...omium.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Ben Chan <benchan@...omium.org>,
	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 5/5] coredump: abort core dump piping only due to a fatal signal

On Fri, Feb 15, 2013 at 7:01 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 02/14, Mandeep Singh Baines wrote:
>>
>> This patch makes wait_for_dump_helpers() not to abort piping the core
>> dump data when the crashing process has received any but a fatal signal
>> (SIGKILL). The rationale is that a crashing process may still receive
>> uninteresting signals such as SIGCHLD when its core dump data is being
>> redirected to a helper application.
>
> You already sent this change in the past ;) and I already reviewed it.
>
> It is not enough and imho not good. Damn, I'll try very much to make the
> patches on weekend...
>
>> -     while ((pipe->readers > 1) && (!signal_pending(current))) {
>> +     while ((pipe->readers > 1) && (!fatal_signal_pending(current))) {
>
> This turns pipe_wait() belowe into the busy-wait loop if signal_pending().

D'oh. Thanks for catching that.

Fixed in v3 by blocking non-fatal signals.

> Not good. And not enough, there are other reasons why coredump can fail
> if the signal is pending.

What other reasons did you have in mind?

Since applying an earlier version of this patch, truncated/missing
coredumps are no longer any issue for us.

Maybe it could fail in binfmt->core_dump().

Could the other reasons be addressed in another patch?

>
>>               wake_up_interruptible_sync(&pipe->wait);
>>               kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
>>               pipe_wait(pipe);
>> +             pipe_unlock(pipe);
>> +             try_to_freeze();
>
> Oh, yes. One of the problems with coredump/signals is freezer. Not sure
> what should we do...
>
> But if we add try_to_freeze() here, we need to add more try_to_freeze's,
> think about dumping the huge core on the slow media.
>

We could add more try_to_freeze()s in the dump_write paths to work
even better with freezer. Do you see any issues with just adding it
here for a start. It fixes the non-slow media case.

Thanks much for reviewing this patch.

Regards,
Mandeep

> Oleg.
>
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