Re: [PATCH 8/9] sched: Fix migrate_disable() vs set_cpus_allowed_ptr()

[Peter Zijlstra]

On Thu, Sep 24, 2020 at 08:59:33PM +0100, Valentin Schneider wrote:
> > @@ -2025,19 +2138,8 @@ static int __set_cpus_allowed_ptr(struct
> >       if (cpumask_test_cpu(task_cpu(p), new_mask))
> >               goto out;
> 
> I think this needs a cancellation of any potential pending migration
> requests. Consider a task P0 running on CPU0:
> 
>    P0                     P1                               P2
> 
>    migrate_disable();
>    <preempt>
>                           set_cpus_allowed_ptr(P0, CPU1);
>                           // waits for completion
>                                                            set_cpus_allowed_ptr(P0, CPU0);
>                                                            // Already good, no waiting for completion
>    <resumes>
>    migrate_enable();
>    // task_cpu(p) allowed, no move_task()
> 
> AIUI in this scenario P1 would stay forever waiting.

Hurmph, looking at it, I think you're right. But I'm fairly sure I did
test that, maybe I just didn't run it long enough to hit the window ...

> I *think* this can be
> cured by making this function slightly more hideous:

It's a real beauty isn't it :-/

> ---
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 01113e6f941f..829334f00f7b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2102,6 +2102,8 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
>                                   u32 flags)
>  {
>         const struct cpumask *cpu_valid_mask = cpu_active_mask;
> +	struct set_affinity_pending *pending;
> +	bool cancel_pending = false;
>         unsigned int dest_cpu;
>         struct rq_flags rf;
>         struct rq *rq;
> @@ -2158,14 +2160,20 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
>         }
> 
>         /* Can the task run on the task's current CPU? If so, we're done */
> -	if (cpumask_test_cpu(task_cpu(p), new_mask))
> +	if (cpumask_test_cpu(task_cpu(p), new_mask)) {
> +		cancel_pending = true;
>                 goto out;
> +	}
> 
>         return move_task(rq, &rf, p, dest_cpu, flags);
> 
>  out:
> +	pending = p->migration_pending;
>         task_rq_unlock(rq, p, &rf);
> 
> +	if (cancel_pending && pending)
> +		complete_all(&pending->done);
> +
>         return ret;
>  }

He who completes pending should also clear ->migration_pending,
otherwise the next caller will be able to observe a dangling pointer.

The other approach is trying to handle that last condition in
move_task(), but I'm quite sure that's going to be aweful too :/