linux-kernel - Re: [tip:locking/core] sched/wait: Fix signal handling in bit wait helpers

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Message-ID: <285059043.27110857.1449839316960.JavaMail.zimbra@redhat.com>
Date:	Fri, 11 Dec 2015 08:08:36 -0500 (EST)
From:	Jan Stancek <jstancek@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Paul Turner <pjt@...gle.com>, NeilBrown <nfbrown@...ell.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...nel.org>,
	Peter Anvin <hpa@...or.com>,
	vladimir murzin <vladimir.murzin@....com>,
	linux-tip-commits@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [tip:locking/core] sched/wait: Fix signal handling in bit wait
 helpers





----- Original Message -----
> From: "Peter Zijlstra" <peterz@...radead.org>
> To: "Paul Turner" <pjt@...gle.com>
> Cc: "NeilBrown" <nfbrown@...ell.com>, "Linus Torvalds" <torvalds@...ux-foundation.org>, "Thomas Gleixner"
> <tglx@...utronix.de>, "LKML" <linux-kernel@...r.kernel.org>, "Mike Galbraith" <efault@....de>, "Ingo Molnar"
> <mingo@...nel.org>, "Peter Anvin" <hpa@...or.com>, "vladimir murzin" <vladimir.murzin@....com>,
> linux-tip-commits@...r.kernel.org, jstancek@...hat.com, "Oleg Nesterov" <oleg@...hat.com>
> Sent: Friday, 11 December, 2015 12:39:59 PM
> Subject: Re: [tip:locking/core] sched/wait: Fix signal handling in bit wait helpers
> 
> On Fri, Dec 11, 2015 at 03:30:33AM -0800, Paul Turner wrote:
> 
> > > Blergh, all I've managed to far is to confuse myself further. Even
> > > something like the original (+- the EINTR) should work when we consider
> > > the looping, even when mixed with an occasional spurious wakeup.
> > >
> > >
> > > int bit_wait()
> > > {
> > >         if (signal_pending_state(current->state, current))
> > >                 return -EINTR;
> > >         schedule();
> > > }
> 
> So I asked Vladimir to test that (simply changing the return from 1 to
> -EINTR) and it made his fail much less likely but it still failed in the
> same way.
> 
> So I'm fairly sure I'm still missing something :/
> 
> > Hugh asked me about this after seeing a crash, here's another exciting
> > way in which the current code breaks -- this one actually quite
> > serious:
> 
> Yep, this got reported by Jan and I did kick myself for that.
> 
> > Peter's proposed follow-up above looks strictly more correct.  We need
> > to evaluate the potential existence of a signal, *after* we return
> > from schedule, but in the context of the state which we previously
> > _entered_ schedule() on.
> > 
> > Reviewed-by: Paul Turner <pjt@...gle.com>
> 
> Right, its maybe a bit overkill, but at this point I'm a tad
> conservative/paranoid.
> 
> Vladimir, Jan could you both please that patch?
> 
>  lkml.kernel.org/r/20151208104712.GJ6356@...ns.programming.kicks-ass.net

This appears to exactly match patch I tested against v4.4-rc4 here:
  http://marc.info/?l=linux-mm&m=144950957622869&w=2

Anyway, I repeated the test with v4.4-rc4-113-g0bd0f1e as base.
Results look good. With patch applied, I can't trigger
"kernel BUG at mm/filemap.c:238!" anymore.

Regards,
Jan

> 
> 
> Thanks!
> 
> 
> 
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