printk deadlock due to double lock attempt on current CPU's runqueue

[Sultan Alsawaf]

On Wed, Nov 10, 2021 at 10:00:35AM +0100, Vincent Guittot wrote:
> Is it the same SCHED_WARN_ON(rq->tmp_alone_branch !=
> &rq->leaf_cfs_rq_list); that generates the deadlock on v5.15 too ?
> 
> one remaining tmp_alone_branch warning has been fixed in v5.15 with
> 2630cde26711 ("sched/fair: Add ancestors of unthrottled undecayed cfs_rq")

I should clarify that I didn't actually reproduce the issue on v5.15; I just saw
that the call chain leading to the deadlock still existed in v5.15 after looking
through the code.

Failing the SCHED_WARN_ON(rq->tmp_alone_branch != &rq->leaf_cfs_rq_list); assert
is extremely rare in my experience, and I don't have a reproducer. It has only
happened once after months of heavy usage (with lots of reboots too, so not with
crazy high uptime).

Sultan