linux-kernel - Re: [PATCH] sched/ext: Suppress warning in __this_cpu

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20250716154901.GA1613200@noisy.programming.kicks-ass.net>
Date: Wed, 16 Jul 2025 17:49:01 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Andrea Righi <arighi@...dia.com>
Cc: Breno Leitao <leitao@...ian.org>, Tejun Heo <tj@...nel.org>,
	David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>, sched-ext@...ts.linux.dev,
	linux-kernel@...r.kernel.org, kernel-team@...a.com,
	jake@...lion.co.uk
Subject: Re: [PATCH] sched/ext: Suppress warning in __this_cpu_write() by
 disabling preemption

On Wed, Jul 16, 2025 at 04:26:23PM +0200, Andrea Righi wrote:
> On Wed, Jul 16, 2025 at 03:36:31PM +0200, Peter Zijlstra wrote:
> > On Wed, Jul 16, 2025 at 03:15:12PM +0200, Andrea Righi wrote:
> > 
> > > The idea is to track the scx callbacks that are invoked with a rq lock held
> > > and, in those cases, store the locked rq. However, some callbacks may also
> > > be invoked from an unlocked context, where no rq is locked and in this case
> > > rq should be NULL.
> > > 
> > > In the latter case, it's acceptable for preemption to remain enabled, but
> > > we still want to explicitly set locked_rq = NULL. If during the execution
> > > of the callback we jump on another CPU, it'd still be in an unlocked state,
> > > so it's locked_rq is still NULL.
> > 
> > Right; but doing superfluous NULL stores seems pointless. So better to
> > avoid the store entirely, rather than making it more expensive and no
> > less pointless, right?
> 
> Right, we can definitely avoid rewriting NULL.
> The following should do the trick.
> 
> Breno, can you give it a try?
> 
> Thanks,
> -Andrea
> 
>  kernel/sched/ext.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index e231450768897..c76d6c9e497b4 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -1290,12 +1290,15 @@ static inline void update_locked_rq(struct rq *rq)
>  	 */
>  	if (rq)
>  		lockdep_assert_rq_held(rq);
> +
> +	WARN_ON_ONCE(preemptible());
>  	__this_cpu_write(scx_locked_rq_state, rq);
>  }

__this_cpu_*() implies that check, which is how we got here ;-)