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
