linux-kernel - Re: [PATCH v2] cpufreq: stats: Add memory barrier to store

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Date:   Wed, 7 Oct 2020 10:49:00 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>
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 <rafael.j.wysocki@...el.com>
> 
> 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@...el.com>
> ---
> 
> 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@...aro.org>

-- 
viresh