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

Peter Zijlstra
2008-12-09T15:00:20Z

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;
> > _