linux-kernel - Re: [PATCHv3] coresight: etm4x: Fix etm4_count race by moving cpuhp callbacks to init

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Message-ID: <46cc76c0-3ed7-9877-6cec-0f9ad6226cef@arm.com>
Date:   Mon, 3 Aug 2020 23:56:23 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     saiprakash.ranjan@...eaurora.org, mathieu.poirier@...aro.org,
        mike.leach@...aro.org, swboyd@...omium.org
Cc:     coresight@...ts.linaro.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCHv3] coresight: etm4x: Fix etm4_count race by moving cpuhp
 callbacks to init

On 07/29/2020 06:13 AM, 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 moving cpuhp callbacks to etm driver init since the
> cpuhp callbacks doesn't have to depend on the etm4_count and can be once
> setup during driver init. Similarly we move cpu_pm notifier registration
> to driver init and completely remove etm4_count usage. Also now we can
> use non cpuslocked version of cpuhp callbacks with this movement.
> 
> Fixes: 9b6a3f3633a5 ("coresight: etmv4: Fix CPU power management setup in probe() function")
> Fixes: 58eb457be028 ("hwtracing/coresight-etm4x: Convert to hotplug state machine")
> Suggested-by: Suzuki K Poulose <suzuki.poulose@....com>
> Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>

Reviewed-by: Suzuki K Poulose <suzuki.poulose@....com>