linux-kernel - Re: [PATCH] coresight: etm4x: Fix etm4

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Date:   Mon, 27 Jul 2020 19:42:37 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     mathieu.poirier@...aro.org, mike.leach@...aro.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, swboyd@...omium.org,
        linux-kernel@...r.kernel.org, coresight@...ts.linaro.org
Subject: Re: [PATCH] coresight: etm4x: Fix etm4_count race using atomic
 variable

On 2020-07-27 15:09, Suzuki K Poulose wrote:
> On 07/27/2020 07:07 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 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@...aro.org>
>> (Mike: Rootcause and context for commit message)
>> Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
> 
> Please could we leave the hotplug notifier installed with the driver
> init and don't worry about this at all ? We bail out early in the
> notifier anyways, if the CPU is not registered with its ETM.
> 

Sure thing, sorry for not taking the cue from earlier discussion.

Thanks,
Sai

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