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

[Suzuki K Poulose]

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@xxxxxxxxxx>
> (Mike: Rootcause and context for commit message)
> Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@xxxxxxxxxxxxxx>

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.

Cheers
Suzuki