Re: [PATCH 1/1] sched: CPU remove deadlock fix

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2008-12-09 at 15:52 +0100, Peter Zijlstra wrote:
> On Tue, 2008-12-09 at 08:47 -0600, Brian King wrote:
> > This patch fixes a possible deadlock scenario in the CPU remove path.
> > migration_call grabs rq->lock, then wakes up everything on rq->migration_queue
> > with the lock held. Then one of the tasks on the migration queue ends up
> > calling tg_shares_up which then also tries to acquire the same rq->lock.
> 
> Looks ok, does lockdep agree?

On second thought, I'm not seeing it at all..

why doesn't every wakeup deadlock?

> > Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
> > ---
> > 
> >  kernel/sched.c |    2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff -puN kernel/sched.c~sched_cpu_down_deadlock_fix kernel/sched.c
> > --- linux-2.6/kernel/sched.c~sched_cpu_down_deadlock_fix	2008-12-09 08:42:09.000000000 -0600
> > +++ linux-2.6-bjking1/kernel/sched.c	2008-12-09 08:42:09.000000000 -0600
> > @@ -6587,7 +6587,9 @@ migration_call(struct notifier_block *nf
> >  			req = list_entry(rq->migration_queue.next,
> >  					 struct migration_req, list);
> >  			list_del_init(&req->list);
> > +			spin_unlock_irq(&rq->lock);
> >  			complete(&req->done);
> > +			spin_lock_irq(&rq->lock);
> >  		}
> >  		spin_unlock_irq(&rq->lock);
> >  		break;
> > _