cpufreq: stats: Add memory barrier to store_reset()

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@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>

diff --git a/drivers/cpufreq/cpufreq_stats.c b/drivers/cpufreq/cpufreq_stats.c
index 8e7d64f..1b1389f 100644 (file)
--- a/drivers/cpufreq/cpufreq_stats.c
+++ b/drivers/cpufreq/cpufreq_stats.c
@@ -47,6 +47,11 @@ static void cpufreq_stats_reset_table(struct cpufreq_stats *stats)
 
        /* 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 cpufreq_policy *policy, char *buf)
 
        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 cpufreq_policy *policy, const char *buf,
         * 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;