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cpufreq: stats: Add memory barrier to store_reset()

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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>
This commit is contained in:
Rafael J. Wysocki 2020-10-06 21:43:43 +02:00
parent 86836bac55
commit efad4240da
1 changed files with 18 additions and 2 deletions
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20
drivers/cpufreq/cpufreq_stats.c
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@ -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;