[PATCH v2] cpufreq: stats: Add memory barrier to store_reset()

[Rafael J. Wysocki]

From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>

There is nothing to prevent the CPU or the compiler from reordering
the writes to stats->reset_time and stats->reset_pending in
store_reset(), in which case the readers of stats->reset_time may see
a stale value.  Moreover, on 32-bit arches the write to reset_time
cannot be completed in one go, so the readers of it may see a
partially updated value in that case.

To prevent that from happening, add a write memory barrier between
the writes to stats->reset_time and stats->reset_pending in
store_reset() and corresponding read memory barrier in the
readers of stats->reset_time.

Fixes: 40c3bd4cfa6f ("cpufreq: stats: Defer stats update to cpufreq_stats_record_transition()")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
---

linux-next material.

-> v2: Pair read and write memory barriers as appropriate.

---
 drivers/cpufreq/cpufreq_stats.c |   20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

Index: linux-pm/drivers/cpufreq/cpufreq_stats.c
===================================================================
--- linux-pm.orig/drivers/cpufreq/cpufreq_stats.c
+++ linux-pm/drivers/cpufreq/cpufreq_stats.c
@@ -47,6 +47,11 @@ static void cpufreq_stats_reset_table(st
 
 	/* Adjust for the time elapsed since reset was requested */
 	WRITE_ONCE(stats->reset_pending, 0);
+	/*
+	 * Prevent the reset_time read from being reordered before the
+	 * reset_pending accesses in cpufreq_stats_record_transition().
+	 */
+	smp_rmb();
 	cpufreq_stats_update(stats, READ_ONCE(stats->reset_time));
 }
 
@@ -71,10 +76,16 @@ static ssize_t show_time_in_state(struct
 
 	for (i = 0; i < stats->state_num; i++) {
 		if (pending) {
-			if (i == stats->last_index)
+			if (i == stats->last_index) {
+				/*
+				 * Prevent the reset_time read from occurring
+				 * before the reset_pending read above.
+				 */
+				smp_rmb();
 				time = get_jiffies_64() - READ_ONCE(stats->reset_time);
-			else
+			} else {
 				time = 0;
+			}
 		} else {
 			time = stats->time_in_state[i];
 			if (i == stats->last_index)
@@ -99,6 +110,11 @@ static ssize_t store_reset(struct cpufre
 	 * avoid races.
 	 */
 	WRITE_ONCE(stats->reset_time, get_jiffies_64());
+	/*
+	 * The memory barrier below is to prevent the readers of reset_time from
+	 * seeing a stale or partially updated value.
+	 */
+	smp_wmb();
 	WRITE_ONCE(stats->reset_pending, 1);
 
 	return count;