Re: [PATCH] coresight: etm4x: Fix etm4_count race using atomic variable

From: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
To: Mathieu Poirier <mathieu.poirier@linaro.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Mike Leach <mike.leach@linaro.org>
Cc: linux-arm-msm@vger.kernel.org, coresight@lists.linaro.org,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	Stephen Boyd <swboyd@chromium.org>
Subject: Re: [PATCH] coresight: etm4x: Fix etm4_count race using atomic variable
Date: Mon, 27 Jul 2020 14:26:00 +0530	[thread overview]
Message-ID: <0541ded117bb2a63481de8c015282b11@codeaurora.org> (raw)
In-Reply-To: <20200727060728.15027-1-saiprakash.ranjan@codeaurora.org>

On 2020-07-27 11:37, Sai Prakash Ranjan wrote:
> etm4_count keeps track of number of ETMv4 registered and on some
> systems, a race is observed on etm4_count variable which can
> lead to multiple calls to cpuhp_setup_state_nocalls_cpuslocked().
> This function internally calls cpuhp_store_callbacks() which
> prevents multiple registrations of callbacks for a given state
> and due to this race, it returns -EBUSY leading to ETM probe
> failures like below.
> 
>  coresight-etm4x: probe of 7040000.etm failed with error -16
> 
> This race can easily be triggered with async probe by setting
> probe type as PROBE_PREFER_ASYNCHRONOUS and with ETM power
> management property "arm,coresight-loses-context-with-cpu".
> 
> Prevent this race by converting etm4_count variable to atomic.
> 
> Fixes: 9b6a3f3633a5 ("coresight: etmv4: Fix CPU power management setup
> in probe() function")
> Fixes: 58eb457be028 ("hwtracing/coresight-etm4x: Convert to hotplug
> state machine")
> Suggested-by: Mike Leach <mike.leach@linaro.org>
> (Mike: Rootcause and context for commit message)
> Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
> ---
>  drivers/hwtracing/coresight/coresight-etm4x.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-etm4x.c
> b/drivers/hwtracing/coresight/coresight-etm4x.c
> index 6d7d2169bfb2..f256ea744c51 100644
> --- a/drivers/hwtracing/coresight/coresight-etm4x.c
> +++ b/drivers/hwtracing/coresight/coresight-etm4x.c
> @@ -49,7 +49,7 @@ MODULE_PARM_DESC(pm_save_enable,
>  	"Save/restore state on power down: 1 = never, 2 = self-hosted");
> 
>  /* The number of ETMv4 currently registered */
> -static int etm4_count;
> +static atomic_t etm4_count;
>  static struct etmv4_drvdata *etmdrvdata[NR_CPUS];
>  static void etm4_set_default_config(struct etmv4_config *config);
>  static int etm4_set_event_filters(struct etmv4_drvdata *drvdata,
> @@ -1403,7 +1403,7 @@ static int etm4_pm_setup_cpuslocked(void)
>  {
>  	int ret;
> 
> -	if (etm4_count++)
> +	if (atomic_inc_return(&etm4_count))
>  		return 0;
> 

Sorry, I messed up here, will send a next version fixing this.

Thanks,
Sai

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