Re: [PATCH] PM / devfreq: Lock devfreq in trans_stat_show

From: Matthias Kaehlcke <mka@chromium.org>
To: Leonard Crestez <leonard.crestez@nxp.com>
Cc: "Artur Świgoń" <a.swigon@partner.samsung.com>,
	linux-pm@vger.kernel.org, "Krzysztof Kozlowski" <krzk@kernel.org>,
	"Lukasz Luba" <l.luba@partner.samsung.com>,
	"Chanwoo Choi" <cw00.choi@samsung.com>,
	"Kyungmin Park" <kyungmin.park@samsung.com>,
	"MyungJoo Ham" <myungjoo.ham@samsung.com>,
	"NXP Linux Team" <linux-imx@nxp.com>,
	"Georgi Djakov" <georgi.djakov@linaro.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] PM / devfreq: Lock devfreq in trans_stat_show
Date: Mon, 23 Sep 2019 16:34:38 -0700	[thread overview]
Message-ID: <20190923233438.GH133864@google.com> (raw)
In-Reply-To: <714675448e7fbf3c930b0dca6fbe54fa5f87211b.1569256001.git.leonard.crestez@nxp.com>

On Mon, Sep 23, 2019 at 07:27:27PM +0300, Leonard Crestez wrote:
> There is no locking in this sysfs show function so stats printing can
> race with a devfreq_update_status called as part of freq switching or
> with initialization.
> 
> Also add an assert in devfreq_update_status to make it clear that lock
> must be held by caller.
> 
> Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
> ---
>  drivers/devfreq/devfreq.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> Changes since v1:
> * Split from series: low-priority bugfix not strictly required for PM QoS
> * Only keep lock during update, release before sprintf
> Link to v1: https://patchwork.kernel.org/patch/11149493/
> 
> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
> index 4c58fbf7d4e4..00fc23fea5b2 100644
> --- a/drivers/devfreq/devfreq.c
> +++ b/drivers/devfreq/devfreq.c
> @@ -206,10 +206,11 @@ int devfreq_update_status(struct devfreq *devfreq, unsigned long freq)
>  {
>  	int lev, prev_lev, ret = 0;
>  	unsigned long cur_time;
>  
>  	cur_time = jiffies;
> +	lockdep_assert_held(&devfreq->lock);
>  
>  	/* Immediately exit if previous_freq is not initialized yet. */
>  	if (!devfreq->previous_freq)
>  		goto out;
>  
> @@ -1507,16 +1508,22 @@ static ssize_t trans_stat_show(struct device *dev,
>  	struct devfreq *devfreq = to_devfreq(dev);
>  	ssize_t len;
>  	int i, j;
>  	unsigned int max_state = devfreq->profile->max_state;
>  
> -	if (!devfreq->stop_polling &&
> -			devfreq_update_status(devfreq, devfreq->previous_freq))
> -		return 0;
>  	if (max_state == 0)
>  		return sprintf(buf, "Not Supported.\n");
>  
> +	/* lock and update */

nit: the comment doesn't add much value, I'd suggest to remove it.

Forgot to comment on this earlier. My Reviewed-by tag still stands, up
to you if you want to re-spin.

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