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

Brian King
2008-12-09T15:08:27Z

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?

I didn't have lockdep enabled when I verified it, but the section of code now
looks like:

spin_lock_irq(&rq->lock);
while (!list_empty(&rq->migration_queue)) {
struct migration_req *req;

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);

So I'm pretty sure lockdep will agree.

-Brian

>
>> 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;
>> _
>

--
Brian King
Linux on Power Virtualization
IBM Linux Technology Center