linux-kernel - Re: 3.14-rc2 XFS backtrace because irqs

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Message-ID: <20140212214411.GQ18016@ZenIV.linux.org.uk>
Date:	Wed, 12 Feb 2014 21:44:11 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Dave Chinner <david@...morbit.com>,
	Dave Jones <davej@...hat.com>,
	Eric Sandeen <sandeen@...deen.net>,
	Linux Kernel <linux-kernel@...r.kernel.org>, xfs@....sgi.com
Subject: Re: 3.14-rc2 XFS backtrace because irqs_disabled.

On Wed, Feb 12, 2014 at 01:32:55PM -0800, Linus Torvalds wrote:
> On Wed, Feb 12, 2014 at 1:14 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > Umm...  What if we delay __sigqueue_free()?  After all, that's where the
> > fat sucker normally comes from.  That way we might get away with much
> > smaller structure on stack...
> 
> Sounds like the RightThing(tm) to do to me, and I don't see why it
> wouldn't work.
> 
> We'd have to teach each user of "dequeue_signal()" to free the siginfo
> thing. Which shouldn't be too bad - I think we've collected all of
> that into generic code, and there isn't the mass or architecture code
> that knows about these things any more. But there are a few odd
> drivers etc and signalfd. I didn't look at what the lifetimes were.

Only signalfd, AFAICS.  And there we'd want to use the same small structure -
it's used in
        do {
                ret = signalfd_dequeue(ctx, &info, nonblock);
                if (unlikely(ret <= 0))
                        break;
                ret = signalfd_copyinfo(siginfo, &info);
                if (ret < 0)
                        break;
                siginfo++;
                total += ret;
                nonblock = 1;
        } while (--count);
and using a smaller struct would actually speed the things up - skips one
copying.  sigqueue would be freed as soon as we'd done signalfd_copyinfo()
(if not by signalfd_copyinfo() itself).

I'll try to put something along those lines together, if you or Oleg don't
do it first.
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