2020-10-06 19:45:58

by Rafael J. Wysocki

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

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;

2020-10-07 06:23:34

by Viresh Kumar

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

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

Acked-by: Viresh Kumar <[email protected]>

--
viresh