linux-kernel - Re: [BUG] 2.6.28-git LOCKDEP: Possible recursive rq->lock

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Date:	Wed, 7 Jan 2009 19:12:33 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <efault@....de>
Subject: Re: [BUG] 2.6.28-git LOCKDEP: Possible recursive rq->lock


* Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com> wrote:

> * Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com> [2009-01-07 22:01:00]:
> 
> > * Peter Zijlstra <a.p.zijlstra@...llo.nl> [2009-01-07 15:28:57]:
> > 
> > > On Wed, 2009-01-07 at 19:50 +0530, Vaidyanathan Srinivasan wrote:
> > > > * Peter Zijlstra <a.p.zijlstra@...llo.nl> [2009-01-07 14:12:43]:
> > > > 
> > > > > On Wed, 2009-01-07 at 17:59 +0530, Vaidyanathan Srinivasan wrote:
> > > > > 
> > > > > > =============================================
> > > > > > [ INFO: possible recursive locking detected ]
> > > > > > 2.6.28-autotest-tip-sv #1
> > > > > > ---------------------------------------------
> > > > > > klogd/5062 is trying to acquire lock:
> > > > > >  (&rq->lock){++..}, at: [<ffffffff8022aca2>] task_rq_lock+0x45/0x7e
> > > > > > 
> > > > > > but task is already holding lock:
> > > > > >  (&rq->lock){++..}, at: [<ffffffff805f7354>] schedule+0x158/0xa31
> > > > > > 
> > > > > > other info that might help us debug this:
> > > > > > 1 lock held by klogd/5062:
> > > > > >  #0:  (&rq->lock){++..}, at: [<ffffffff805f7354>] schedule+0x158/0xa31
> > > > > > 
> > > > > > stack backtrace:
> > > > > > Pid: 5062, comm: klogd Not tainted 2.6.28-autotest-tip-sv #1
> > > > > > Call Trace:
> > > > > >  [<ffffffff80259ef1>] __lock_acquire+0xeb9/0x16a4
> > > > > >  [<ffffffff8025a6c0>] ? __lock_acquire+0x1688/0x16a4
> > > > > >  [<ffffffff8025a761>] lock_acquire+0x85/0xa9
> > > > > >  [<ffffffff8022aca2>] ? task_rq_lock+0x45/0x7e
> > > > > >  [<ffffffff805fa4d4>] _spin_lock+0x31/0x66
> > > > > >  [<ffffffff8022aca2>] ? task_rq_lock+0x45/0x7e
> > > > > >  [<ffffffff8022aca2>] task_rq_lock+0x45/0x7e
> > > > > >  [<ffffffff80233363>] try_to_wake_up+0x88/0x27a
> > > > > >  [<ffffffff80233581>] wake_up_process+0x10/0x12
> > > > > >  [<ffffffff805f775c>] schedule+0x560/0xa31
> > > > > 
> > > > > I'd be most curious to know where in schedule we are.
> > > > 
> > > > ok, we are in sched.c:3777
> > > > 
> > > >                 double_unlock_balance(this_rq, busiest);
> > > >                 if (active_balance)
> > > > >>>>>>>>>>>              wake_up_process(busiest->migration_thread);
> > > > 
> > > >         } else  
> > > > 
> > > > In active balance in newidle.  This implies sched_mc was 2 at that time.
> > > > let me trace this and debug further.
> > > 
> > > How about something like this? Strictly speaking we'll not deadlock,
> > > because ttwu will not be able to place the migration task on our rq, but
> > > since the code can deal with both rqs getting unlocked, this seems the
> > > easiest way out.
> > 
> > Hi Peter,
> > 
> > I agree.  Unlocking this_rq is an easy way out.  Thanks for the
> > suggestion.  I have moved the unlock and lock withing the if
> > condition.
> > 
> > --Vaidy
> > 
> >         sched: bug fix -- do not call ttwu while holding rq->lock
> >     
> >         When sched_mc=2 wake_up_process() is called on busiest_rq
> >         while holding this_rq lock in load_balance_newidle()
> >         Though this will not deadlock, this is a lockdep warning
> >         and the situation is easily solved by releasing the this_rq
> >         lock at this point in code
> >     
> >     Signed-off-by: Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
> > 
> > diff --git a/kernel/sched.c b/kernel/sched.c
> > index 71a054f..703a669 100644
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -3773,8 +3773,12 @@ redo:
> >                 }
> >  
> >                 double_unlock_balance(this_rq, busiest);
> > -               if (active_balance)
> > +               if (active_balance) {
> > +                       /*  Should not call ttwu while holding a rq->lock */
> > +                       spin_unlock(&this_rq->lock);
> >                         wake_up_process(busiest->migration_thread);
> > +                       spin_lock(&this_rq->lock);
> > +               }
> >  
> >         } else
> >                 sd->nr_balance_failed = 0;
> 
> 
> Hi Peter and Ingo,
> 
> The above fix seem to have fixed the lockdep warning.  Please include
> in sched-tip for further testing and later push to mainline.

already in tip/sched/urgent, thanks guys!

	Ingo
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