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

Peter Zijlstra
2008-12-09T15:12:18Z

On Tue, 2008-12-09 at 15:59 +0100, Peter Zijlstra wrote:
> 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?

because I'm blind...

void __wake_up(wait_queue_head_t *q, unsigned int mode,
int nr_exclusive, void *key)
{
unsigned long flags;

spin_lock_irqsave(&q->lock, flags);
__wake_up_common(q, mode, nr_exclusive, 0, key);
spin_unlock_irqrestore(&q->lock, flags);
}

that's q->lock, not rq->lock...

> > > 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;
> > > _