linux-kernel - Re: rcu_preempt detected stalls.

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Message-ID: <20141027211329.GJ5718@linux.vnet.ibm.com>
Date:	Mon, 27 Oct 2014 14:13:29 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>, htejun@...il.com
Subject: Re: rcu_preempt detected stalls.

On Fri, Oct 24, 2014 at 12:39:15PM -0400, Sasha Levin wrote:
> On 10/24/2014 12:13 PM, Paul E. McKenney wrote:
> > On Fri, Oct 24, 2014 at 08:28:40AM -0400, Sasha Levin wrote:
> >> > On 10/23/2014 03:58 PM, Paul E. McKenney wrote:
> >>> > > On Thu, Oct 23, 2014 at 02:55:43PM -0400, Sasha Levin wrote:
> >>>>> > >> > On 10/23/2014 02:39 PM, Paul E. McKenney wrote:
> >>>>>>> > >>> > > On Tue, Oct 14, 2014 at 10:35:10PM -0400, Sasha Levin wrote:
> >>>>>>>>> > >>>> > >> On 10/13/2014 01:35 PM, Dave Jones wrote:
> >>>>>>>>>>> > >>>>> > >>> oday in "rcu stall while fuzzing" news:
> >>>>>>>>>>> > >>>>> > >>>
> >>>>>>>>>>> > >>>>> > >>> INFO: rcu_preempt detected stalls on CPUs/tasks:
> >>>>>>>>>>> > >>>>> > >>> 	Tasks blocked on level-0 rcu_node (CPUs 0-3): P766 P646
> >>>>>>>>>>> > >>>>> > >>> 	Tasks blocked on level-0 rcu_node (CPUs 0-3): P766 P646
> >>>>>>>>>>> > >>>>> > >>> 	(detected by 0, t=6502 jiffies, g=75434, c=75433, q=0)
> >>>>>>>>> > >>>> > >>
> >>>>>>>>> > >>>> > >> I've complained about RCU stalls couple days ago (in a different context)
> >>>>>>>>> > >>>> > >> on -next. I guess whatever causing them made it into Linus's tree?
> >>>>>>>>> > >>>> > >>
> >>>>>>>>> > >>>> > >> https://lkml.org/lkml/2014/10/11/64
> >>>>>>> > >>> > > 
> >>>>>>> > >>> > > And on that one, I must confess that I don't see where the RCU read-side
> >>>>>>> > >>> > > critical section might be.
> >>>>>>> > >>> > > 
> >>>>>>> > >>> > > Hmmm...  Maybe someone forgot to put an rcu_read_unlock() somewhere.
> >>>>>>> > >>> > > Can you reproduce this with CONFIG_PROVE_RCU=y?
> >>>>> > >> > 
> >>>>> > >> > Paul, if that was directed to me - Yes, I see stalls with CONFIG_PROVE_RCU
> >>>>> > >> > set and nothing else is showing up before/after that.
> >>> > > Indeed it was directed to you.  ;-)
> >>> > > 
> >>> > > Does the following crude diagnostic patch turn up anything?
> >> > 
> >> > Nope, seeing stalls but not seeing that pr_err() you added.
> > OK, color me confused.  Could you please send me the full dmesg or a
> > pointer to it?
> 
> Attached.

Thank you!  I would complain about the FAULT_INJECTION messages, but
they don't appear to be happening all that frequently.

The stack dumps do look different here.  I suspect that this is a real
issue in the VM code.

							Thanx, Paul

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