linux-kernel - Re: [PATCH v2 2/2] perf/arm-ccn: Remove broken race mitigation

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Date:   Thu, 4 Apr 2019 11:46:50 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     robin.murphy@....com, will.deacon@....com
Cc:     mark.rutland@....com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        bigeasy@...utronix.de, peterz@...radead.org,
        clabbe.montjoie@...il.com, Meng.Li@...driver.com
Subject: Re: [PATCH v2 2/2] perf/arm-ccn: Remove broken race mitigation

Hi Robin,

On 03/04/2019 18:10, Robin Murphy wrote:
> Like arm-cci, arm-ccn has the same issue of disabling preemption around
> operations which can take mutexes. Again, remove the definite bug by
> simply not trying to fight the theoretical races. And since we are
> touching the hotplug handling code, take the opportunity to streamline
> it, as there's really no need to store a full-sized cpumask to keep
> track of a single CPU ID.
> 
> Signed-off-by: Robin Murphy <robin.murphy@....com>
> ---
>   drivers/perf/arm-ccn.c | 23 +++++++++++------------
>   1 file changed, 11 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/perf/arm-ccn.c b/drivers/perf/arm-ccn.c
> index 2ae76026e947..a0214308b0cd 100644
> --- a/drivers/perf/arm-ccn.c
> +++ b/drivers/perf/arm-ccn.c

>   	/* Pick one CPU which we will use to collect data from CCN... */
> -	cpumask_set_cpu(get_cpu(), &ccn->dt.cpu);
> +	ccn->dt.cpu = raw_smp_processor_id();
>   
>   	/* Also make sure that the overflow interrupt is handled by this CPU */
>   	if (ccn->irq) {
> -		err = irq_set_affinity_hint(ccn->irq, &ccn->dt.cpu);
> +		err = irq_set_affinity_hint(ccn->irq, cpumask_of(ccn->dt.cpu));
>   		if (err) {
>   			dev_err(ccn->dev, "Failed to set interrupt affinity!\n");
>   			goto error_set_affinity;
>   		}
>   	}
>   
> +	cpuhp_state_add_instance_nocalls(CPUHP_AP_PERF_ARM_CCN_ONLINE,
> +					 &ccn->dt.node);
> +
>   	err = perf_pmu_register(&ccn->dt.pmu, name, -1);
>   	if (err)
>   		goto error_pmu_register;

Should we not remove the above instance, in case we fail to register
the PMU ? Similarly for the CCI driver, we may have to reset the g_cci_pmu
if we fail.

Cheers
Suzuki


>   
> -	cpuhp_state_add_instance_nocalls(CPUHP_AP_PERF_ARM_CCN_ONLINE,
> -					 &ccn->dt.node);
> -	put_cpu();
>   	return 0;
>   
>   error_pmu_register:
>   error_set_affinity:
> -	put_cpu();
>   error_choose_name:
>   	ida_simple_remove(&arm_ccn_pmu_ida, ccn->dt.id);
>   	for (i = 0; i < ccn->num_xps; i++)
>