linux-kernel - Re: [PATCH -next v2] power/qos: annotate data races in pm_qos_*

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Date:   Tue, 3 Mar 2020 23:37:07 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Qian Cai <cai@....pw>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Marco Elver <elver@...gle.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next v2] power/qos: annotate data races in pm_qos_*_value

On Wed, Feb 26, 2020 at 2:58 AM Qian Cai <cai@....pw> wrote:
>
> The target_value field in struct pm_qos_constraints is used for lockless
> access to the effective constraint value of a given QoS list, so the
> readers of it cannot expect it to always reflect the most recent
> effective constraint value.  However, they can and do expect it to be
> equal to a valid effective constraint value computed at a certain time
> in the past (event though it may not be the most recent one), so add
> READ|WRITE_ONCE() annotations around the target_value accesses to
> prevent the compiler from possibly causing that expectation to be unmet
> by generating code in an exceptionally convoluted way.
>
> Signed-off-by: Qian Cai <cai@....pw>
> ---
>
> v2: borrow the commit log from Rafael.
>
>  kernel/power/qos.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/power/qos.c b/kernel/power/qos.c
> index 32927682bcc4..db0bed2cae26 100644
> --- a/kernel/power/qos.c
> +++ b/kernel/power/qos.c
> @@ -52,7 +52,7 @@ static DEFINE_SPINLOCK(pm_qos_lock);
>   */
>  s32 pm_qos_read_value(struct pm_qos_constraints *c)
>  {
> -       return c->target_value;
> +       return READ_ONCE(c->target_value);
>  }
>
>  static int pm_qos_get_value(struct pm_qos_constraints *c)
> @@ -75,7 +75,7 @@ static int pm_qos_get_value(struct pm_qos_constraints *c)
>
>  static void pm_qos_set_value(struct pm_qos_constraints *c, s32 value)
>  {
> -       c->target_value = value;
> +       WRITE_ONCE(c->target_value, value);
>  }
>
>  /**
> --

Applied as 5.7 material, thanks!