[PATCH] sched: Fix 32bit race in sched_clock_remote()

[Peter Zijlstra <peterz@infradead.org>]

Thomas spotted a nasty 32bit race in sched_clock_remote() after way too
many hours of debugging weirdness.

What happens is that sched_clock_remote() does regular machine word
reads of sched_clock_data::clock; this appears safe since we use
cmpxchg64() to update the variable and any half-read value would
trigger a retry.

Except we don't validate the new value 'val' in the same way! Thus we
can propagate non-atomic read errors into the clock value.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Debugged-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 kernel/sched/clock.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index c685e31..7042ef7 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -170,6 +170,21 @@ static u64 sched_clock_local(struct sched_clock_data *scd)
 	return clock;
 }
 
+#ifndef CONFIG_64BIT
+/*
+ * 32bit machines can't atomically read a u64 except using cmpxchg64()
+ */
+static inline u64 scd_read_clock(struct sched_clock_data *scd)
+{
+	return cmpxchg64(&scd->clock, 0, 0);
+}
+#else
+static inline u64 scd_read_clock(struct sched_clock_data *scd)
+{
+	return scd->clock;
+}
+#endif
+
 static u64 sched_clock_remote(struct sched_clock_data *scd)
 {
 	struct sched_clock_data *my_scd = this_scd();
@@ -178,8 +193,8 @@ static u64 sched_clock_remote(struct sched_clock_data *scd)
 
 	sched_clock_local(my_scd);
 again:
-	this_clock = my_scd->clock;
-	remote_clock = scd->clock;
+	this_clock = scd_clock_read(my_scd);
+	remote_clock = scd_clock_read(scd);
 
 	/*
 	 * Use the opportunity that we have both locks