linux-kernel - Re: New RCU related warning due to rcu_preempt

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Message-ID: <CA+1xoqc9ETMg3WxbNg_+KzEeYFVEgSwpWNPCdhEMDW2P=vQ+nQ@mail.gmail.com>
Date:	Wed, 18 Apr 2012 07:29:19 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Dave Jones <davej@...hat.com>,
	"linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>
Subject: Re: New RCU related warning due to rcu_preempt_depth() changes

On Tue, Apr 17, 2012 at 6:45 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Tue, Apr 17, 2012 at 08:53:16AM -0700, Paul E. McKenney wrote:
>> On Tue, Apr 17, 2012 at 05:36:59PM +0200, Sasha Levin wrote:
>> > On Tue, Apr 17, 2012 at 5:05 PM, Paul E. McKenney
>> > <paulmck@...ux.vnet.ibm.com> wrote:
>> > > On Tue, Apr 17, 2012 at 10:42:47AM +0200, Sasha Levin wrote:
>> > >> Hi Paul,
>> > >>
>> > >> It looks like commit 7298b03 ("rcu: Move __rcu_read_lock() and
>> > >> __rcu_read_unlock() to per-CPU variables") is causing the following
>> > >> warning (I've added the extra fields on the second line):
>> > >>
>> > >> [   77.330920] BUG: sleeping function called from invalid context at
>> > >> mm/memory.c:3933
>> > >> [   77.336571] in_atomic(): 0, irqs_disabled(): 0, preempt count: 0,
>> > >> preempt offset: 0, rcu depth: 1, pid: 5669, name: trinity
>> > >> [   77.344135] no locks held by trinity/5669.
>> > >> [   77.349644] Pid: 5669, comm: trinity Tainted: G        W
>> > >> 3.4.0-rc3-next-20120417-sasha-dirty #83
>> > >> [   77.354401] Call Trace:
>> > >> [   77.355956]  [<ffffffff810e83f3>] __might_sleep+0x1f3/0x210
>> > >> [   77.358811]  [<ffffffff81198eaf>] might_fault+0x2f/0xa0
>> > >> [   77.361997]  [<ffffffff810e3228>] schedule_tail+0x88/0xb0
>> > >> [   77.364671]  [<ffffffff826a01d3>] ret_from_fork+0x13/0x80
>> > >>
>> > >> As you can see, rcu_preempt_depth() returns 1 when running in that
>> > >> context, which looks pretty odd.
>> > >
>> > > Ouch!!!
>> > >
>> > > So it looks like I missed a place where I need to save and restore
>> > > the new per-CPU rcu_read_lock_nesting and rcu_read_unlock_special
>> > > variables.  My (probably hopelessly naive) guess is that I need to add
>> > > a rcu_switch_from() and rcu_switch_to() into schedule_tail(), but to
>> > > make rcu_switch_from() take the task_struct pointer as an argument,
>> > > passing in prev.
>> > >
>> > > Does this make sense, or am I still missing something here?
>> >
>> > I've let the test run for a bit more, and it appears that I'm getting
>> > this warning from lots of different sources, would this
>> > schedule_tail() fix all of them?
>>
>> If I understand the failure correctly, yes.  If the task switches without
>> RCU paying attention, the nesting count for both the outgoing and the
>> incoming tasks can get messed up.  The messed-up counts could easily
>> cause problems downstream.
>>
>> Of course, there might well be additional bugs.
>>
>> I will put a speculative patch together and send it along.
>
> And here it is, testing just started.
>
>                                                        Thanx, Paul
>
> ------------------------------------------------------------------------
>
> rcu: Add RCU context switching to schedule_tail()
>
> The new rcu_read_lock_nesting and rcu_read_unlock_special per-CPU
> variables must be saved and restored at every context switch, including
> those involving schedule_tail().  This commit therefore adds the saving
> and restoring to schedul_tail().
>
> Reported-by: Sasha Levin <levinsasha928@...il.com>
> Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>

Looks good here.

Thanks.
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