[PATCH RT 1/2] hrtimer: fix logic for when grabbing softirq_expiry_lock can be elided

From: zanussi@kernel.org
To: LKML <linux-kernel@vger.kernel.org>,
	linux-rt-users <linux-rt-users@vger.kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Carsten Emde <C.Emde@osadl.org>, John Kacur <jkacur@redhat.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Daniel Wagner <wagi@monom.org>,
	Clark Williams <williams@redhat.com>,
	Tom Zanussi <zanussi@kernel.org>,
	Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Subject: [PATCH RT 1/2] hrtimer: fix logic for when grabbing softirq_expiry_lock can be elided
Date: Tue, 28 Apr 2020 14:52:55 -0500	[thread overview]
Message-ID: <e342baf3a1e01475391e9171b45ef5a09e85ed9e.1588103561.git.zanussi@kernel.org> (raw)
In-Reply-To: <cover.1588103561.git.zanussi@kernel.org>
In-Reply-To: <cover.1588103561.git.zanussi@kernel.org>

From: Rasmus Villemoes <rasmus.villemoes@prevas.dk>

v4.19.115-rt50-rc1 stable review patch.
If anyone has any objections, please let me know.

-----------


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@prevas.dk>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Tom Zanussi <zanussi@kernel.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..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);
 	}
-- 
2.17.1

