Messages in this thread | | | Date | Wed, 14 May 2014 12:26:02 +0200 | From | Peter Zijlstra <> | Subject | Re: sched: hang in migrate_swap |
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On Wed, May 14, 2014 at 02:21:04PM +0400, Kirill Tkhai wrote: > > > 14.05.2014, 14:14, "Peter Zijlstra" <peterz@infradead.org>: > > On Wed, May 14, 2014 at 01:42:32PM +0400, Kirill Tkhai wrote: > > > >> Peter, do we have to queue stop works orderly? > >> > >> Is there is not a possibility, when two pair of works queued different on > >> different cpus? > >> > >> kernel/stop_machine.c | 10 ++++++++-- > >> 1 file changed, 8 insertions(+), 2 deletions(-) > >> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c > >> index b6b67ec..29e221b 100644 > >> --- a/kernel/stop_machine.c > >> +++ b/kernel/stop_machine.c > >> @@ -250,8 +250,14 @@ struct irq_cpu_stop_queue_work_info { > >> static void irq_cpu_stop_queue_work(void *arg) > >> { > >> struct irq_cpu_stop_queue_work_info *info = arg; > >> - cpu_stop_queue_work(info->cpu1, info->work1); > >> - cpu_stop_queue_work(info->cpu2, info->work2); > >> + > >> + if (info->cpu1 < info->cpu2) { > >> + cpu_stop_queue_work(info->cpu1, info->work1); > >> + cpu_stop_queue_work(info->cpu2, info->work2); > >> + } else { > >> + cpu_stop_queue_work(info->cpu2, info->work2); > >> + cpu_stop_queue_work(info->cpu1, info->work1); > >> + } > >> } > > > > I'm not sure, we already send the IPI to the first cpu of the pair, so > > supposing we have 4 cpus, and get 4 pairs like: > > > > 0,1 1,2 2,3 3,0 > > > > That would result in IPIs to 0, 1, 2, and 0 again, and since the IPI > > function is serialized I don't immediately see a way for this to > > deadlock. > > It's about stop_two_cpus(), I have a distrust about other users of stop task: > > queue_stop_cpus_work() queues work consequentially: > > 0 1 2 4 > > stop_two_cpus() may queue: > > 1 0 > > Looks like, stop thread on 0th and on 1th are waiting for wrong works.
so we serialize stop_cpus_work() vs stop_two_cpus() with an l/g lock.
Ah, but stop_cpus_work() only holds the global lock over queueing, it doesn't wait for completion, that might indeed cause a problem.
Also, since its two different cpus queueing, the ordered queue doesn't really matter, you can still interleave the all and two sets and get into this state. [unhandled content-type:application/pgp-signature] | |