linux-kernel - Re: [PATCH 1/2] sched_ext: Track currently locked rq

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: <qxulb3ckm256bltfep45iac3vifv342o24654ulh4zt6shvg5j@grp7crx56rk3>
Date: Wed, 16 Jul 2025 03:47:38 -0700
From: Breno Leitao <leitao@...ian.org>
To: Andrea Righi <arighi@...dia.com>
Cc: Tejun Heo <tj@...nel.org>, David Vernet <void@...ifault.com>, 
	Changwoo Min <changwoo@...lia.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched_ext: Track currently locked rq

Hello Andrea,

On Tue, Jul 15, 2025 at 07:20:28PM +0200, Andrea Righi wrote:
> > On Tue, Apr 22, 2025 at 10:26:32AM +0200, Andrea Righi wrote:

> > 
> > > +		lockdep_assert_rq_held(rq);
> > > +	__this_cpu_write(locked_rq, rq);
> > 
> > This is hitting the following BUG() on some of my debug kernels:
> > 
> > 	BUG: using __this_cpu_write() in preemptible [00000000] code: scx_layered_6-9/68770
> > 
> > I have lockdep enabled, and I don't see the assert above. I am wondering
> > if rq is locked but preemption continues to be enabled (!?)
> 
> Interesting. And it makes sense, because we may have callbacks called from
> a preemptible context (especially when rq == NULL).
> 
> I think we can just put a preempt_disable() / preempt_enable() around
> __this_cpu_write(). If we jump to another CPU during the callback it's
> fine, since we would track the rq state on the other CPU with its own local
> variable. And if we were able to jump there, it means that preemption was
> disabled as well.

First of all thanks for the suggestion!

What about a patch like the following:

commit 9ed31e914181ec8f2d0b4484c42b00b6794661b9
Author: Breno Leitao <leitao@...ian.org>
Date:   Wed Jul 16 03:10:59 2025 -0700

    sched/ext: Suppress warning in __this_cpu_write() by disabling preemption
    
    __this_cpu_write() emits a warning if used with preemption enabled.
    
    Function update_locked_rq() might be called with preemption enabled,
    which causes the following warning:
    
            BUG: using __this_cpu_write() in preemptible [00000000] code: scx_layered_6-9/68770
    
    Disable preemption around the __this_cpu_write() call in
    update_locked_rq() to suppress the warning, without affecting behavior.
    
    If preemption triggers a jump to another CPU during the callback it's
    fine, since we would track the rq state on the other CPU with its own
    local variable.
    
    Suggested-by: Andrea Righi <arighi@...dia.com>
    Signed-off-by: Breno Leitao <leitao@...ian.org>
    Fixes: 18853ba782bef ("sched_ext: Track currently locked rq")

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index b498d867ba210..24fcbd7331f73 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -1258,7 +1258,14 @@ static inline void update_locked_rq(struct rq *rq)
 	 */
 	if (rq)
 		lockdep_assert_rq_held(rq);
+	/*
+	 * __this_cpu_write() emits a warning when used with preemption enabled.
+	 * While there's no functional issue if the callback runs on another
+	 * CPU, we disable preemption here solely to suppress that warning.
+	 */
+	preempt_disable();
 	__this_cpu_write(locked_rq, rq);
+	preempt_enable();
 }
 
 /*