Re: [PATCH v4 07/16] locking/rwsem: Implement lock handoff to prevent lock starvation

[Peter Zijlstra]

On Tue, Apr 16, 2019 at 02:16:11PM -0400, Waiman Long wrote:

> >> @@ -324,6 +364,12 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
> >>  		adjustment -= RWSEM_FLAG_WAITERS;
> >>  	}
> >>  
> >> +	/*
> >> +	 * Clear the handoff flag
> >> +	 */
> > Right, but that is a trivial comment in the 'increment i' style, it
> > clearly states what the code does, but completely fails to elucidate the
> > code.
> >
> > Maybe:
> >
> > 	/*
> > 	 * When we've woken a reader, we no longer need to force writers
> > 	 * to give up the lock and we can clear HANDOFF.
> > 	 */
> >
> > And I suppose this is required if we were the pickup of the handoff set
> > above, but is there a guarantee that the HANDOFF was not set by a
> > writer?
> 
> I can change the comment. The handoff bit is always cleared in
> rwsem_try_write_lock() when the lock is successfully acquire. Will add a
> comment to document that.

That doesn't help much, because it drops ->wait_lock between setting it
and acquiring it. So the read-acquire can interleave.

I _think_ it works, but I'm having trouble explaining how exactly. I
think because readers don't spin yet and thus wakeups abide by queue
order.

And the other way around should have (write) spinners terminate the
moment they see HANDOFF set by a readers, but I'm not immediately seeing
that either.

I'll continue staring at that.