[PATCH] sched: Fix data-race in wakeup

[Mel Gorman]

On Tue, Nov 17, 2020 at 09:30:16AM +0100, Peter Zijlstra wrote:
> > sched_psi_wake_requeue can probably stay with the other three fields
> > given they are under the rq lock but sched_remote_wakeup needs to move
> > out.
> 
> I _think_ we can move the bit into the unserialized section below.
> 
> It's a bit cheecky, but it should work I think because the only time we
> actually use this bit, we're guaranteed the task isn't actually running,
> so current doesn't exist.
> 

Putting the bit there has the added advantage that if the bit existed
on its own that it would be very special in terms of how it should be
treated. Adding a bit adjacent to it would be potentially hazardous.

> ---
> Subject: sched: Fix data-race in wakeup
> From: Peter Zijlstra <peterz at infradead.org>
> Date: Tue Nov 17 09:08:41 CET 2020
>  
> <SNIP>
> 
> Fixes: c6e7bd7afaeb ("sched/core: Optimize ttwu() spinning on p->on_cpu")
> Reported-by: Mel Gorman <mgorman at techsingularity.net>
> Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>

Thanks, testing completed successfully! With the suggested alternative
comment above sched_remote_wakeup;

Reviewed-by: Mel Gorman <mgorman at techsingularity.net>

-- 
Mel Gorman
SUSE Labs