Re: [PATCH v2] sched_ext: Fix lock imbalance in dispatch_to_local_dsq()

[Tejun Heo]

Hello, Andrea.

Thanks for debugging this.

On Fri, Jan 24, 2025 at 08:24:25AM +0100, Andrea Righi wrote:
...
> Fix this by correctly assuming that task is still in src_rq in this
> specific scenario.

But I find this a bit misleading. It's more that we didn't do anything and
thus didn't switch the held lock and the assumption that the unlock path
makes is wrong.

...
> +	} else {
> +		/*
> +		 * Otherwise, if dequeue wins the race, we no longer have
> +		 * exclusive ownership of the task and we must keep it in
> +		 * its original @src_dsq.
> +		 */
> +		dst_rq = src_rq;

Both the code and explanation are confusing to me. This is working around
the incorrect assumption the following block is making - that we'd be
locking $dsq_rq when control reach that point. Can you instead add new
variable $locked_rq which tracks which rq is currently locked and then use
that in the unlock path? It starts with $rq and set to $src_rq when
switching to that and then set to $dst_rq in the migration block. Then, the
unlock path can test @rq against $locked_rq. scx_dsq_move() uses a similar
approach for reference.

Thanks.

-- 
tejun