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    SubjectRe: sched: hang in migrate_swap
    Date


    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.
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