linux-kernel - [PATCH -rt] hrtimer: fix logic for when grabbing softirq_expiry

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Message-Id: <20200428144026.5882-1-rasmus.villemoes@prevas.dk>
Date:   Tue, 28 Apr 2020 16:40:26 +0200
From:   Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To:     linux-kernel@...r.kernel.org
Cc:     linux-rt-users@...r.kernel.org,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Tom Zanussi <zanussi@...nel.org>,
        Julien Grall <julien.grall@....com>,
        Daniel Wagner <wagi@...om.org>, John Kacur <jkacur@...hat.com>
Subject: [PATCH -rt] hrtimer: fix logic for when grabbing softirq_expiry_lock can be elided

Commit

  hrtimer: Add a missing bracket and hide `migration_base' on !SMP

which is 47b6de0b7f22 in 5.2-rt and 40aae5708e7a in 4.19-rt,
inadvertently changed the logic from base != &migration_base to base
== &migration_base.

On !CONFIG_SMP, the effect was to effectively always elide this
lock/unlock pair (since is_migration_base() is unconditionally false),
which for me consistently causes lockups during reboot, and reportedly
also often causes a hang during boot.

Adding this logical negation (or, what is effectively the same thing
on !CONFIG_SMP, reverting the above commit as well as "hrtimer:
Prevent using hrtimer_grab_expiry_lock() on migration_base") fixes
that lockup.

Fixes: 40aae5708e7a (hrtimer: Add a missing bracket and hide `migration_base' on !SMP) # 4.19-rt
Fixes: 47b6de0b7f22 (hrtimer: Add a missing bracket and hide `migration_base' on !SMP) # 5.2-rt
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@...vas.dk>
---
Something like this? I wasn't sure what Fixes: tag(s) to include, if
any. It's quite possible the same fix is needed on earlier -rt
kernels, I didn't check.

 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..c3966c090246 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 && !is_migration_base(base)) {
 		spin_lock(&base->cpu_base->softirq_expiry_lock);
 		spin_unlock(&base->cpu_base->softirq_expiry_lock);
 	}
-- 
2.23.0