sched/clock: Fix hotplug crash

Mike reported that he could trigger the WARN_ON_ONCE() in
set_sched_clock_stable() using hotplug.

This exposed a fundamental problem with the interface, we should never
mark the TSC stable if we ever find it to be unstable. Therefore
set_sched_clock_stable() is a broken interface.

The reason it existed is that not having it is a pain, it means all
relevant architecture code needs to call clear_sched_clock_stable()
where appropriate.

Of the three architectures that select HAVE_UNSTABLE_SCHED_CLOCK ia64
and parisc are trivial in that they never called
set_sched_clock_stable(), so add an unconditional call to
clear_sched_clock_stable() to them.

For x86 the story is a lot more involved, and what this patch tries to
do is ensure we preserve the status quo. So even is Cyrix or Transmeta
have usable TSC they never called set_sched_clock_stable() so they now
get an explicit mark unstable.

Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 9881b024b7d7 ("sched/clock: Delay switching sched_clock to stable")
Link: http://lkml.kernel.org/r/20170119133633.GB6536@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 8 insertions, 21 deletions

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index 7713b2b53f61..ad64efe41722 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -83,8 +83,15 @@ void sched_clock_init(void)
 }
 
 #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
+/*
+ * We must start with !__sched_clock_stable because the unstable -> stable
+ * transition is accurate, while the stable -> unstable transition is not.
+ *
+ * Similarly we start with __sched_clock_stable_early, thereby assuming we
+ * will become stable, such that there's only a single 1 -> 0 transition.
+ */
 static DEFINE_STATIC_KEY_FALSE(__sched_clock_stable);
-static int __sched_clock_stable_early;
+static int __sched_clock_stable_early = 1;
 
 /*
  * We want: ktime_get_ns() + gtod_offset == sched_clock() + raw_offset
@@ -132,24 +139,6 @@ static void __set_sched_clock_stable(void)
 	tick_dep_clear(TICK_DEP_BIT_CLOCK_UNSTABLE);
 }
 
-void set_sched_clock_stable(void)
-{
-	__sched_clock_stable_early = 1;
-
-	smp_mb(); /* matches sched_clock_init_late() */
-
-	/*
-	 * This really should only be called early (before
-	 * sched_clock_init_late()) when guestimating our sched_clock() is
-	 * solid.
-	 *
-	 * After that we test stability and we can negate our guess using
-	 * clear_sched_clock_stable, possibly from a watchdog.
-	 */
-	if (WARN_ON_ONCE(sched_clock_running == 2))
-		__set_sched_clock_stable();
-}
-
 static void __clear_sched_clock_stable(struct work_struct *work)
 {
 	struct sched_clock_data *scd = this_scd();
@@ -199,8 +188,6 @@ void sched_clock_init_late(void)
 
 	if (__sched_clock_stable_early)
 		__set_sched_clock_stable();
-	else
-		__clear_sched_clock_stable(NULL);
 }
 
 /*