Re: [PATCH RFC 03/26] sched: Replace spin_unlock_wait() with lock/unlock pair

[Arnd Bergmann]

On Fri, Jun 30, 2017 at 2:01 AM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> There is no agreed-upon definition of spin_unlock_wait()'s semantics,
> and it appears that all callers could do just as well with a lock/unlock
> pair.  This commit therefore replaces the spin_unlock_wait() call in
> do_task_dead() with spin_lock() followed immediately by spin_unlock().
> This should be safe from a performance perspective because the lock is
> this tasks ->pi_lock, and this is called only after the task exits.
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index e91138fcde86..6dea3d9728c8 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3461,7 +3461,8 @@ void __noreturn do_task_dead(void)
>          * is held by try_to_wake_up()
>          */
>         smp_mb();
> -       raw_spin_unlock_wait(&current->pi_lock);
> +       raw_spin_lock(&current->pi_lock);
> +       raw_spin_unlock(&current->pi_lock);

Does the raw_spin_lock()/raw_spin_unlock() imply an smp_mb() or stronger?
Maybe it would be clearer to remove the extra barrier if so.

     Arnd