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

From: Suzuki K Poulose
Date: Mon Jul 27 2020 - 05:35:06 EST

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.