linux-kernel - Re: Linux 3.19-rc3

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Date:	Tue, 6 Jan 2015 03:07:30 -0800
From:	Kent Overstreet <kmo@...erainc.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Sedat Dilek <sedat.dilek@...il.com>,
	Dave Jones <davej@...emonkey.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, Chris Mason <clm@...com>
Subject: Re: Linux 3.19-rc3

On Tue, Jan 06, 2015 at 12:01:12PM +0100, Peter Zijlstra wrote:
> On Tue, Jan 06, 2015 at 11:18:04AM +0100, Sedat Dilek wrote:
> > On Tue, Jan 6, 2015 at 11:06 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > > On Tue, Jan 06, 2015 at 10:57:19AM +0100, Sedat Dilek wrote:
> > >> [   88.028739]  [<ffffffff8124433f>] aio_read_events+0x4f/0x2d0
> > >>
> > >
> > > Ah, that one. Chris Mason and Kent Overstreet were looking at that one.
> > > I'm not touching the AIO code either ;-)
> > 
> > I know, I was so excited when I see nearly the same output.
> > 
> > Can you tell me why people see "similiar" problems in different areas?
> 
> Because the debug check is new :-) It's a pattern that should not be
> used but mostly works most of the times.
> 
> > [  181.397024] WARNING: CPU: 0 PID: 2872 at kernel/sched/core.c:7303
> > __might_sleep+0xbd/0xd0()
> > [  181.397028] do not call blocking ops when !TASK_RUNNING; state=1
> > set at [<ffffffff810b83bd>] prepare_to_wait_event+0x5d/0x110
> > 
> > With similiar buzzwords... namely...
> > 
> > mutex_lock_nested
> > prepare_to_wait(_event)
> > __might_sleep
> > 
> > I am asking myself... Where is the real root cause - in sched/core?
> > Fix one single place VS. fix the impact at several other places?
> 
> No, the root cause is nesting sleep primitives, this is not fixable in
> the one place, both prepare_to_wait and mutex_lock are using
> task_struct::state, they have to, no way around it.

No, it's completely possible to construct a prepare_to_wait() that doesn't
require messing with the task state. Had it for years.

http://evilpiepirate.org/git/linux-bcache.git/log/?h=aio_ring_fix
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