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SubjectRe: [PATCH] sched/core: Add missing completion for affine_move_task() waiters
Hi,

On Fri, Nov 13, 2020 at 11:24:14AM +0000, Valentin Schneider wrote:

> Qian reported that some fuzzer issuing sched_setaffinity() ends up stuck on
> a wait_for_completion(). The problematic pattern seems to be:
> affine_move_task()
> // task_running() case
> stop_one_cpu();
> wait_for_completion(&pending->done);
>
> Combined with, on the stopper side:
>
> migration_cpu_stop()
> // Task moved between unlocks and scheduling the stopper
> task_rq(p) != rq &&
> // task_running() case
> dest_cpu >= 0
>
> => no complete_all()
>
> This can happen with both PREEMPT and !PREEMPT, although !PREEMPT should
> be more likely to see this given the targeted task has a much bigger window
> to block and be woken up elsewhere before the stopper runs.
>
> Make migration_cpu_stop() always look at pending affinity requests; signal
> their completion if the stopper hits a rq mismatch but the task is
> still within its allowed mask. When Migrate-Disable isn't involved, this
> matches the previous set_cpus_allowed_ptr() vs migration_cpu_stop()
> behaviour.
>
> Link: https://lore.kernel.org/lkml/8b62fd1ad1b18def27f18e2ee2df3ff5b36d0762.camel@redhat.com
> Fixes: 6d337eab041d ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()")
> Reported-by: Qian Cai <cai@redhat.com>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> ---
> kernel/sched/core.c | 13 ++++++++++++-
> 1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 02076e6d3792..fad0a8e62aca 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1923,7 +1923,7 @@ static int migration_cpu_stop(void *data)
> else
> p->wake_cpu = dest_cpu;
>
> - } else if (dest_cpu < 0) {
> + } else if (dest_cpu < 0 || pending) {
> /*
> * This happens when we get migrated between migrate_enable()'s
> * preempt_enable() and scheduling the stopper task. At that
> @@ -1933,6 +1933,17 @@ static int migration_cpu_stop(void *data)
> * more likely.
> */
>
> + /*
> + * The task moved before the stopper got to run. We're holding
> + * ->pi_lock, so the allowed mask is stable - if it got
> + * somewhere allowed, we're done.
> + */
> + if (pending && cpumask_test_cpu(task_cpu(p), p->cpus_ptr)) {
> + p->migration_pending = NULL;
> + complete = true;
> + goto out;
> + }
> +
> /*
> * When this was migrate_enable() but we no longer have an
> * @pending, a concurrent SCA 'fixed' things and we should be
> --
> 2.27.0

Oh, I did not receive this patch from 'ouwen210@hotmail.com'
account. Checked that you sent the patch to that mail address
from web. If 'ouwen210' is not a good mail account name(I have
used this name since 2002), I will change to use this one(Now
is smooth enough and can go to lkml).

Thanks,
Tao

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