[tip:sched/urgent] sched/clock: Fix clear_sched_clock_stable() preempt wobbly

[tip-bot for Peter Zijlstra <tipbot@zytor.com>]

Commit-ID:  71fdb70eb48784c1f28cdf2e67c4c587dd7f2594
Gitweb:     http://git.kernel.org/tip/71fdb70eb48784c1f28cdf2e67c4c587dd7f2594
Author:     Peter Zijlstra <peterz@infradead.org>
AuthorDate: Mon, 13 Mar 2017 13:46:21 +0100
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 23 Mar 2017 07:31:48 +0100

sched/clock: Fix clear_sched_clock_stable() preempt wobbly

Paul reported a problems with clear_sched_clock_stable(). Since we run
all of __clear_sched_clock_stable() from workqueue context, there's a
preempt problem.

Solve it by only running the static_key_disable() from workqueue.

Reported-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: fweisbec@gmail.com
Link: http://lkml.kernel.org/r/20170313124621.GA3328@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/clock.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index a08795e..fec0f58 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -141,7 +141,14 @@ static void __set_sched_clock_stable(void)
 	tick_dep_clear(TICK_DEP_BIT_CLOCK_UNSTABLE);
 }
 
-static void __clear_sched_clock_stable(struct work_struct *work)
+static void __sched_clock_work(struct work_struct *work)
+{
+	static_branch_disable(&__sched_clock_stable);
+}
+
+static DECLARE_WORK(sched_clock_work, __sched_clock_work);
+
+static void __clear_sched_clock_stable(void)
 {
 	struct sched_clock_data *scd = this_scd();
 
@@ -160,11 +167,11 @@ static void __clear_sched_clock_stable(struct work_struct *work)
 			scd->tick_gtod, gtod_offset,
 			scd->tick_raw,  raw_offset);
 
-	static_branch_disable(&__sched_clock_stable);
 	tick_dep_set(TICK_DEP_BIT_CLOCK_UNSTABLE);
-}
 
-static DECLARE_WORK(sched_clock_work, __clear_sched_clock_stable);
+	if (sched_clock_stable())
+		schedule_work(&sched_clock_work);
+}
 
 void clear_sched_clock_stable(void)
 {
@@ -173,7 +180,7 @@ void clear_sched_clock_stable(void)
 	smp_mb(); /* matches sched_clock_init_late() */
 
 	if (sched_clock_running == 2)
-		schedule_work(&sched_clock_work);
+		__clear_sched_clock_stable();
 }
 
 void sched_clock_init_late(void)