Re: [PATCH v4 09/16] locking/rwsem: Ensure an RT task will not spin on reader

[Peter Zijlstra]

On Wed, Apr 17, 2019 at 02:47:07PM -0400, Waiman Long wrote:
> >> @@ -566,13 +573,28 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
> >>  		}
> >>  
> >>  		/*
> >> -		 * When there's no owner, we might have preempted between the
> >> -		 * owner acquiring the lock and setting the owner field. If
> >> -		 * we're an RT task that will live-lock because we won't let
> >> -		 * the owner complete.
> >> +		 * An RT task cannot do optimistic spinning if it cannot
> >> +		 * be sure the lock holder is running or live-lock may
> >> +		 * happen if the current task and the lock holder happen
> >> +		 * to run in the same CPU.
> >> +		 *
> >> +		 * When there's no owner or is reader-owned, an RT task
> >> +		 * will stop spinning if the owner state is not a writer
> >> +		 * at the previous iteration of the loop. This allows the
> >> +		 * RT task to recheck if the task that steals the lock is
> >> +		 * a spinnable writer. If so, it can keeps on spinning.
> >> +		 *
> >> +		 * If the owner is a writer, the need_resched() check is
> >> +		 * done inside rwsem_spin_on_owner(). If the owner is not
> >> +		 * a writer, need_resched() check needs to be done here.
> >>  		 */
> >> -		if (!sem->owner && (need_resched() || rt_task(current)))
> >> -			break;
> >> +		if (owner_state != OWNER_WRITER) {
> >> +			if (need_resched())
> >> +				break;
> >> +			if (is_rt_task && (prev_owner_state != OWNER_WRITER))
> >> +				break;
> >> +		}
> >> +		prev_owner_state = owner_state;
> >>  
> >>  		/*
> >>  		 * The cpu_relax() call is a compiler barrier which forces
> > This patch confuses me mightily. I mean, I see what it does, but I can't
> > figure out why. The Changelog is just one big source of confusion.
> 
> Sorry for confusing you. If count and owner are separate, there is a
> time lag where the owner is NULL, but the lock is not free yet.

Right.

> Similarly, the lock could be free but another task may have stolen the
> lock if the waiter bit isn't set.

> In the former case,

(free)

> an extra iteration gives it more time for the lock holder to release
> the lock.


> In the latter case,

(stolen)

> if the new lock owner is a writer and set owner in time,
> the RT task can keep on spinning. Will clarify that in the commit log
> and the comment.

Blergh.. so by going around once extra, you hope ->owner will be set
again and we keep spinning. And this is actually measurable.

Yuck yuck yuck. I much prefer getting rid of that hole, as you do later
on in the series, that would avoid this complecity. Let me continue
reading...