From: Rafael J. Wysocki <[email protected]>
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().
Fixes: 40c3bd4cfa6f ("cpufreq: stats: Defer stats update to
cpufreq_stats_record_transition()")
Signed-off-by: Rafael J. Wysocki <[email protected]>
---
I couldn't convince myself that it was OK to leave the code as it was.
linux-next material.
---
drivers/cpufreq/cpufreq_stats.c | 7 +++++++
1 file changed, 7 insertions(+)
Index: linux-pm/drivers/cpufreq/cpufreq_stats.c
===================================================================
--- linux-pm.orig/drivers/cpufreq/cpufreq_stats.c
+++ linux-pm/drivers/cpufreq/cpufreq_stats.c
@@ -99,6 +99,13 @@ 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. Note that they both access
+ * reset_time only if reset_pending is 1, so corresponding read barriers
+ * are not needed.
+ */
+ smp_wmb();
WRITE_ONCE(stats->reset_pending, 1);
return count;
