Re: [PATCH tip/sched/core] Add comments to aid in safer usage of swake_up.

[Steven Rostedt]

On Wed, 14 Jun 2017 09:10:15 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> Now let's make it simpler. I'll even add the READ_ONCE and WRITE_ONCE
> where applicable.
> 
> 
> 	CPU0				CPU1
> 	----				----
> 				LOCK(A)
> 
>  LOCK(B)
> 				 WRITE_ONCE(X, INIT)
> 
> 				 (the cpu may postpone writing X)
> 
> 				 (the cpu can fetch wq list here)
>   list_add(wq, q)
> 
>  UNLOCK(B)
> 
>  (the cpu may fetch old value of X)
> 
> 				 (write of X happens here)
> 
>  if (READ_ONCE(X) != init)
>    schedule();
> 
> 				UNLOCK(A)
> 
> 				 if (list_empty(wq))
> 				   return;
> 
> Tell me again how the READ_ONCE() and WRITE_ONCE() helps in this
> scenario?
> 
> Because we are using spinlocks, this wont be an issue for most
> architectures. The bug happens if the fetching of the list_empty()
> leaks into before the UNLOCK(A).
> 
> If the reading/writing of the list and the reading/writing of gp_flags
> gets reversed in either direction by the CPU, then we have a problem.

FYI..

Both sides need a memory barrier. Otherwise, even with a memory barrier
on CPU1 we can still have:


	CPU0				CPU1
	----				----

				LOCK(A)
 LOCK(B)

 list_add(wq, q)

 (cpu waits to write wq list)

 (cpu fetches X)

				 WRITE_ONCE(X, INIT)

				UNLOCK(A)

				smp_mb();

				if (list_empty(wq))
				   return;

 (cpu writes wq list)

 UNLOCK(B)

 if (READ_ONCE(X) != INIT)
   schedule()


Luckily for us, there is a memory barrier on CPU0. In
prepare_to_swait() we have:

	raw_spin_lock_irqsave(&q->lock, flags);
	__prepare_to_swait(q, wait);
	set_current_state(state);
	raw_spin_unlock_irqrestore(&q->lock, flags);

And that set_current_state() call includes a memory barrier, which will
prevent the above from happening, as the addition to the wq list must
be flushed before fetching X.

I still strongly believe that the swait_active() requires a memory
barrier.

-- Steve