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Subject: Re: [PATCH 5/5] perf/arm_dsu: Fix CPU hotplug races
To:     Suzuki K Poulose <suzuki.poulose@arm.com>, will.deacon@arm.com,
        mark.rutland@arm.com
Cc:     linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        peterz@infradead.org, tglx@linutronix.de, bigeasy@linutronix.de
References: <cover.1549299188.git.robin.murphy@arm.com>
 <d376f6bbd5804616b38145780789785b19a37543.1549299188.git.robin.murphy@arm.com>
 <9fcf862c-0d07-cece-852f-6c09128aa0c5@arm.com>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <84bdba44-9ce1-906c-0575-f931435de886@arm.com>
Date:   Tue, 5 Feb 2019 13:04:23 +0000
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Hi Suzuki,

On 05/02/2019 11:40, Suzuki K Poulose wrote:
> On 04/02/2019 17:09, Robin Murphy wrote:
>> Like other system PMUs which associate themselves with an arbitrary CPU
>> for housekeeping purposes, arm_dsu has a race between registering the
>> hotplug notifier and registering the PMU device, such that the hotplug
>> niotifier can potentially fire and attempt to migrate the PMU context
>> before the latter is valid. This is easily resolved by inhibiting
>> hotplug until both the notifier and PMU device are successfully set up.
>>
>> For the same reason, also suppress any synchronous notifier calls in the
>> cleanup path if PMU registration fails.
>>
>> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> 
> Should we add :
> 
> Fixes: commit 7520fa99246dade7ab6 ("perf: ARM DynamIQ Shared Unit PMU 
> support")
> 
> Either way:
> 
> Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>

Thanks for the reviews! I think this patch is worthwhile for cleanliness 
and consistency, but since it's neither self-contained (with the 
dependency on #4) nor "fix[ing] a real bug that bothers people" I'm not 
convinced it really deserves backporting - unlike the preemption issue, 
actually getting a crash from this race in practice is unlikely enough 
that it would probably require some determined, deliberate effort to 
trigger it with just the right conditions.

Cheers,
Robin.