linux-kernel - Re: [PATCH v2 2/3] freezer: do not send a fake signal to a PF

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Date:	Thu, 28 Feb 2013 07:39:51 -0800
From:	Mandeep Singh Baines <msb@...omium.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Neil Horman <nhorman@...hat.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] freezer: do not send a fake signal to a
 PF_DUMPCORE thread

On Wed, Feb 27, 2013 at 10:08 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 02/26, Mandeep Singh Baines wrote:
>>
>> >> Change freeze_task() to check PF_DUMPCORE along with PF_KTHREAD. We
>> >> need to recheck PF_DUMPCORE under ->siglock to avoid the race with
>> >> zap_threads() which can set this flag right before we take the lock.
>> >>
>> >
>> > Won't this prevent suspend?
>
> Hmm. I guess you mean that pipe_write() can hang in pipe_wait() if the
> user-space handler was already freezed... Damn, and I even mentioned
> this race when we discussed this 2 weeks ago.
>
> I need to think, but most probably you are right, and we need another
> solution...
>
>> You'd rather have reliable suspend than coredumps that aren't
>> truncated so you need to set TIF_SIGPENDING to break waits in the
>> dump_write path.
>
> Oh, I agree. In this case the necessary changes look simple.
>
>> static void wait_for_dump_helpers(struct file *file)
>> {
>>         struct pipe_inode_info *pipe;
>>
>>         pipe = file->f_path.dentry->d_inode->i_pipe;
>>
>>         pipe_lock(pipe);
>>         pipe->readers++;
>>         pipe->writers--;
>>
>>         while (pipe->readers > 1) {
>>                 unsigned long flags;
>>
>>                 wake_up_interruptible_sync(&pipe->wait);
>>                 kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
>>                 pipe_wait(pipe);
>>
>>                 pipe_unlock(pipe);
>>                 try_to_freeze();
>>                 pipe_lock(pipe);
>>
>>                 if (fatal_signal_pending(current))
>>                         break;
>>
>>                 /* Clear fake signal from freeze_task(). */
>>                 spin_lock_irqsave(&current->sighand->siglock, flags);
>>                 recalc_sigpending();
>>                 spin_unlock_irqrestore(&current->sighand->siglock, flags);
>
> IIRC, this is what you added into your tree. But note that
> recalc_sigpending() is wrong, exactly because (say) SIGCHLD can
> be pending if it was sent before we set SIGNAL_GROUP_COREDUMP.
>
> So this code needs something like
>
>         spin_lock_irq(siglock);
>         if (!fatal_signal_pending)
>                 clear_thread_flag(TIF_SIGPENDING);
>         spin_unlock_irq(siglock);
>
> Or we need to change recalc_sigpending() to check SIGNAL_GROUP_COREDUMP
> or PF_DUMPCORE. I'd like to avoid this, but perhaps we have to do this...
>
> (Btw, this is offtopic, but whatever we do 3/3 still looks like a nice
>  cleanup to me, although it probably needs more changes)
>
>> What do you think? That would fix most cases. You'll still get a
>> truncated core if you were to receive the signal during pipe_write or
>> something.
>
> Let me think a bit...
>
> Right now I can only say that personally I do not really like the
> idea to fix wait_for_dump_helpers() but not pipe_write(). I mean,
> if pipe_write() can fail due to freezing(), then why should we care
> about wait_for_dump_helpers() ? Let them all fail, suspend is not
> that often.
>

I agree. This does seem like a minor problem since its likely very
rare. Probably not worth fixing unless there is an elegant,
non-invasive solution.

As long as suspend is reliable, a very small chance of a truncated
core dumps is OK.

Regards,
Mandeep

> Or we should try to make everything freezer-friendly. But if
> freeze_task() sets TIF_SIGPENDING then we need the ugly "retry"
> logic in dump_write()... Not good.
>
> Thanks Mandeep. If you have other ideas please tell me ;)
>
> Oleg.
>
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