linux-kernel - Re: [RESEND PATCH 3/6] perf/x86/intel/uncore: Extract codes of box ref/unref

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Date:   Wed, 8 May 2019 13:51:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     kan.liang@...ux.intel.com
Cc:     tglx@...utronix.de, mingo@...hat.com, linux-kernel@...r.kernel.org,
        acme@...nel.org, eranian@...gle.com, ak@...ux.intel.com
Subject: Re: [RESEND PATCH 3/6] perf/x86/intel/uncore: Extract codes of box
 ref/unref

On Tue, Apr 30, 2019 at 05:53:45PM -0700, kan.liang@...ux.intel.com wrote:
> +static void uncore_box_unref(struct intel_uncore_type **types, int id)
>  {
> +	struct intel_uncore_type *type;
>  	struct intel_uncore_pmu *pmu;
>  	struct intel_uncore_box *box;
> +	int i;
> +
> +	for (; *types; types++) {
> +		type = *types;
> +		pmu = type->pmus;
> +		for (i = 0; i < type->num_boxes; i++, pmu++) {
> +			box = pmu->boxes[id];
> +			if (box && atomic_dec_return(&box->refcnt) == 0)
> +				uncore_box_exit(box);
> +		}
> +	}
> +}

> +static int uncore_box_ref(struct intel_uncore_type **types,
> +			  int id, unsigned int cpu)
>  {
> +	struct intel_uncore_type *type;
>  	struct intel_uncore_pmu *pmu;
>  	struct intel_uncore_box *box;
> +	int i, ret;
>  
> +	ret = allocate_boxes(types, id, cpu);
>  	if (ret)
>  		return ret;
>  
> @@ -1232,11 +1238,22 @@ static int uncore_event_cpu_online(unsigned int cpu)
>  		type = *types;
>  		pmu = type->pmus;
>  		for (i = 0; i < type->num_boxes; i++, pmu++) {
> +			box = pmu->boxes[id];
>  			if (box && atomic_inc_return(&box->refcnt) == 1)
>  				uncore_box_init(box);
>  		}
>  	}
> +	return 0;
> +}

This relies on all online/offline events to be globally serialized, and
they are. But that does make me wonder why we're using atomic_t.

Without the serialization there is an online-online race where the first
online sets the refcount to 1, lets the second online continue without
the first having completed the init().

Anyway, the code isn't wrong, just weird.