linux-kernel - Re: [RESEND][v2][PATCH] Fix a race between try_to_wake

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Date:   Mon, 5 Sep 2016 18:37:10 +1000
From:   Balbir Singh <bsingharora@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Nicholas Piggin <nicholas.piggin@...il.com>,
        Alexey Kardashevskiy <aik@...abs.ru>
Subject: Re: [RESEND][v2][PATCH] Fix a race between try_to_wake_up() and a
 woken up task

On Mon, Sep 5, 2016 at 5:48 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Sep 05, 2016 at 05:14:19PM +1000, Benjamin Herrenschmidt wrote:
>> On Mon, 2016-09-05 at 13:16 +1000, Balbir Singh wrote:
>>
>>  .../...
>> >
>> > Cc: Peter Zijlstra <peterz@...radead.org>
>> > Cc: Nicholas Piggin <npiggin@...il.com>
>>
>> Acked-by: Benjamin Herrenschmidt <benh@...nel.crashing.org>
>>
>> > Signed-off-by: Balbir Singh <bsingharora@...il.com>
>> > ---
>> >  kernel/sched/core.c | 11 +++++++++++
>> >  1 file changed, 11 insertions(+)
>> >
>> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> > index 2a906f2..582c684 100644
>> > --- a/kernel/sched/core.c
>> > +++ b/kernel/sched/core.c
>> > @@ -2016,6 +2016,17 @@ try_to_wake_up(struct task_struct *p, unsigned
>> > int state, int wake_flags)
>> >     success = 1; /* we're going to change ->state */
>> >     cpu = task_cpu(p);
>> >
>> > +   /*
>> > +    * Ensure we see on_rq and p_state consistently
>> > +    *
>> > +    * For example in __rwsem_down_write_failed(), we have
>> > +    *    [S] ->on_rq = 1                           [L] ->state
>> > +    *    MB                                         RMB
>> > +    *    [S] ->state = TASK_UNINTERRUPTIBLE        [L] ->on_rq
>> > +    * In the absence of the RMB p->on_rq can be observed to be 0
>> > +    * and we end up spinning indefinitely in while (p->on_cpu)
>> > +    */
>
> So I did replace that comment with the one I proposed earlier. I checked
> a fair number of architectures and many did not have an obvious barrier
> in switch_to(). So that is not something we can rely on, nor do we need
> to I think.
>

Thanks for the comment edit and thanks for letting us know.

Balbir Singh