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

From: Viresh Kumar
Date: Wed Oct 07 2020 - 01:19:06 EST


On 06-10-20, 21:43, Rafael J. Wysocki wrote:
> 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;

Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>

--
viresh