linux-kernel - Re: [PATCH RT 10/30] hrtimer: Prevent using hrtimer_grab_expiry_lock() on migration

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Date:   Mon, 27 Apr 2020 14:26:52 -0500
From:   Tom Zanussi <zanussi@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc:     linux-kernel@...r.kernel.org,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        John Kacur <jkacur@...hat.com>,
        Julia Cartwright <julia@...com>,
        Daniel Wagner <wagi@...om.org>,
        Julien Grall <julien.grall@....com>
Subject: Re: [PATCH RT 10/30] hrtimer: Prevent using
 hrtimer_grab_expiry_lock() on migration_base

On Mon, 2020-04-27 at 15:06 -0400, Steven Rostedt wrote:
> On Mon, 27 Apr 2020 15:10:00 +0200
> Rasmus Villemoes <rasmus.villemoes@...vas.dk> wrote:
> 
> > However, building with their rescue initrd and booting that, the
> > board
> > would consistently hang during reboot. Sometimes I would get lines
> > like
> > 
> > [   72.956630] sched: RT throttling activated
> > [   72.973769] lanx: port 1(lan1) entered disabled state
> > [   73.000401] lanx: port 2(lan2) entered disabled state
> > [   73.974951] lanx: port 3(lan3) entered disabled state
> > [   73.997473] lanx: port 4(lan4) entered disabled state
> > [   74.968006] lanx: port 5(lan5) entered disabled state
> > 
> > other times there would be no output, but the board was still
> > hanging.
> > Reverting
> > 
> > b1a471ec4df1 - hrtimer: Prevent using hrtimer_grab_expiry_lock() on
> > migration_base
> > 40aae5708e7a - hrtimer: Add a missing bracket and hide
> > `migration_base'
> > on !SMP
> > 
> > on top of v4.19.94-rt39 makes that problem go away, i.e. the board
> > reboots as expected.
> > 
> > The board is a 32 bit powerpc (mpc8309) !SMP. Any ideas what I can
> > do to
> > debug this further?
> 
> Thanks Rasmus for looking into this. Tom now maintains 4.19-rt.
> 
> Tom, care to pull in these patches on top of 4.19-rt?
> 

Those patches are already in 4.19-rt - he's saying that reverting them
fixes the problem.

I'm guessing that the assumption of base or base->cpu_base always being
non-NULL in those patches might be wrong.  If so, the below patch
should fix the problem:

Subject: [PATCH] hrtimer: Add back base and base->cpu_base checks in
 hrtimer_grab_expiry_lock()

4.19 commit b1a471ec4df1 [hrtimer: Prevent using
hrtimer_grab_expiry_lock() on migration_base] removed the NULL checks
for timer->base and timer->base->cpu_base on the assumption that
they're always non-NULL.  That assumption is apparently not to be
true, so add the checks back.

Signed-off-by: Tom Zanussi <zanussi@...nel.org>
---
 kernel/time/hrtimer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index e54a95de8b79..6f20cf23008b 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -953,7 +953,7 @@ void hrtimer_grab_expiry_lock(const struct hrtimer *timer)
 {
 	struct hrtimer_clock_base *base = READ_ONCE(timer->base);
 
-	if (timer->is_soft && is_migration_base(base)) {
+	if (timer->is_soft && base && base->cpu_base && is_migration_base(base)) {
 		spin_lock(&base->cpu_base->softirq_expiry_lock);
 		spin_unlock(&base->cpu_base->softirq_expiry_lock);
 	}
-- 
2.17.1


> -- Steve