Re: [PATCH] sched/completion: completion_done() should serialize with complete()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Feb 12, 2015 at 08:59:13PM +0100, Oleg Nesterov wrote:
> Commit de30ec47302c "Remove unnecessary ->wait.lock serialization when
> reading completion state" was not correct, without lock/unlock the code
> like stop_machine_from_inactive_cpu()
> 
> 	while (!completion_done())
> 		cpu_relax();
> 
> can return before complete() finishes its spin_unlock() which writes to
> this memory. And spin_unlock_wait().
> 
> While at it, change try_wait_for_completion() to use READ_ONCE().
> 
> Reported-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Reported-by: Davidlohr Bueso <dave@stgolabs.net>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

So I am having some difficulty reproducing the original problem, but
the patch passes rcutorture testing.  So...

Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> --- x/kernel/sched/completion.c
> +++ x/kernel/sched/completion.c
> @@ -274,7 +274,7 @@ bool try_wait_for_completion(struct comp
>  	 * first without taking the lock so we can
>  	 * return early in the blocking case.
>  	 */
> -	if (!ACCESS_ONCE(x->done))
> +	if (!READ_ONCE(x->done))
>  		return 0;
> 
>  	spin_lock_irqsave(&x->wait.lock, flags);
> @@ -297,6 +297,11 @@ EXPORT_SYMBOL(try_wait_for_completion);
>   */
>  bool completion_done(struct completion *x)
>  {
> -	return !!ACCESS_ONCE(x->done);
> +	if (!READ_ONCE(x->done))
> +		return false;
> +
> +	smp_rmb();
> +	spin_unlock_wait(&x->wait.lock);
> +	return true;
>  }
>  EXPORT_SYMBOL(completion_done);
>