linux-kernel - Re: [RFC PATCH 5/5] arm: perf: Use FIQ to handle PMU events.

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Date:	Mon, 19 Jan 2015 17:48:18 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Daniel Thompson <daniel.thompson@...aro.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	patches@...aro.org, linaro-kernel@...ts.linaro.org,
	John Stultz <john.stultz@...aro.org>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Dirk Behme <dirk.behme@...bosch.com>,
	Daniel Drake <drake@...lessm.com>,
	Dmitry Pervushin <dpervushin@...il.com>,
	Tim Sander <tim@...eglstein.org>,
	Stephen Boyd <sboyd@...eaurora.org>
Subject: Re: [RFC PATCH 5/5] arm: perf: Use FIQ to handle PMU events.

On Tue, Jan 13, 2015 at 04:35:31PM +0000, Daniel Thompson wrote:
> +/*
> + * This handler is called *unconditionally* from the default NMI/FIQ
> + * handler. The irq may not be anything to do with us so the main
> + * job of this function is to figure out if the irq passed in is ours
> + * or not.
> + */
> +void cpu_pmu_handle_fiq(int irq)
> +{
> +	int cpu = smp_processor_id();

This can be either debug_smp_processor_id() or raw_smp_processor_id().
raw_smp_processor_id() is fine from FIQ contexts, as seems to be
debug_smp_processor_id(), but only because we guarantee that
irqs_disabled() in there will be true.

> +
> +	if (irq != get_cpu_var(cpu_pmu_irqs))
> +		return;

get_cpu_var() needs put_cpu_var() to undo its effects.  get_cpu_var()
calls preempt_disable(), which calls into lockdep...  I think we
determined that was fine last time we went digging?  put_cpu_var()
would call preempt_enable() which I'd hope would be safe in FIQ/NMI
contexts?

> +
> +	(void)armpmu_dispatch_irq(irq,
> +				  get_cpu_ptr(&cpu_pmu->hw_events->percpu_pmu));

Again, get_cpu_xxx() needs to be balanced with a put_cpu_xxx().


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