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    SubjectRe: [BISECTED] rcu_sched self-detected stall since 3.17
    Sorry again for the huge delay.

    And all I can say is that I am all confused.

    On 12/01, Peter Zijlstra wrote:
    >
    > On Fri, Nov 20, 2015 at 03:35:38PM +0000, Vladimir Murzin wrote:
    > > commit 743162013d40ca612b4cb53d3a200dff2d9ab26e
    > > Author: NeilBrown <neilb@suse.de>
    > > Date: Mon Jul 7 15:16:04 2014 +1000

    That patch still looks correct to me.

    > > and if I apply following diff I don't see stalls anymore.
    > >
    > > diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
    > > index a104879..2d68cdb 100644
    > > --- a/kernel/sched/wait.c
    > > +++ b/kernel/sched/wait.c
    > > @@ -514,9 +514,10 @@ EXPORT_SYMBOL(bit_wait);
    > >
    > > __sched int bit_wait_io(void *word)
    > > {
    > > + io_schedule();
    > > +
    > > if (signal_pending_state(current->state, current))
    > > return 1;
    > > - io_schedule();
    > > return 0;
    > > }
    > > EXPORT_SYMBOL(bit_wait_io);

    I can't understand why this change helps. But note that it actually removes
    the signal_pending_state() check from bit_wait_io(), current->state is always
    TASK_RUNNING after return from schedule(), signal_pending_state() will always
    return zero.

    This means that after this change wait_on_page_bit_killable() will spin in a
    busy-wait loop if the caller is killed.

    > The reason this is broken is that schedule() will no-op when there is a
    > pending signal, while raising a signal will also issue a wakeup.

    But why this is wrong? We should notice signal_pending_state() on the next
    iteration.

    > Thus the right thing to do is check for the signal state after,

    I think this check should work on both sides. The only difference is that
    you obviously can't use current->state after schedule().

    I still can't understand the problem.

    Oleg.



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