linux-kernel - [PATCH] tick-sched: avoid a maybe-uninitialized warning

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Date:   Mon,  9 Apr 2018 14:23:33 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] tick-sched: avoid a maybe-uninitialized warning

The use of bitfields seems to confuse gcc, leading to a false-positive
warning in all compiler versions:

kernel/time/tick-sched.c: In function 'tick_nohz_idle_exit':
kernel/time/tick-sched.c:538:2: error: 'now' may be used uninitialized in this function [-Werror=maybe-uninitialized]

This introduces a temporary variable to track the flags so gcc
doesn't have to evaluate twice, eliminating the code path that
leads to the warning.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85301
Fixes: 1cae544d42d2 ("nohz: Gather tick_sched booleans under a common flag field")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 kernel/time/tick-sched.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 7eef9431ca24..646645e981f9 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1143,6 +1143,7 @@ void tick_nohz_idle_restart_tick(void)
 void tick_nohz_idle_exit(void)
 {
 	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
+	bool idle_active, tick_stopped;
 	ktime_t now;
 
 	local_irq_disable();
@@ -1151,14 +1152,16 @@ void tick_nohz_idle_exit(void)
 	WARN_ON_ONCE(ts->timer_expires_base);
 
 	ts->inidle = 0;
+	idle_active = ts->idle_active;
+	tick_stopped = ts->tick_stopped;
 
-	if (ts->idle_active || ts->tick_stopped)
+	if (idle_active || tick_stopped)
 		now = ktime_get();
 
-	if (ts->idle_active)
+	if (idle_active)
 		tick_nohz_stop_idle(ts, now);
 
-	if (ts->tick_stopped)
+	if (tick_stopped)
 		__tick_nohz_idle_restart_tick(ts, now);
 
 	local_irq_enable();
-- 
2.9.0