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() 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 <[email protected]>
---
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;
