linux-kernel - Re: [RFC PATCH] perf/smmuv3: Fix shared interrupt handling

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Message-ID: <20200624125045.GC6270@willie-the-truck>
Date:   Wed, 24 Jun 2020 13:50:46 +0100
From:   Will Deacon <will@...nel.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     mark.rutland@....com, tuanphan@...amperecomputing.com,
        john.garry@...wei.com, linux-kernel@...r.kernel.org,
        shameerali.kolothum.thodi@...wei.com, harb@...erecomputing.com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH] perf/smmuv3: Fix shared interrupt handling

On Wed, Jun 24, 2020 at 12:48:14PM +0100, Robin Murphy wrote:
> On 2020-04-08 17:49, Robin Murphy wrote:
> > IRQF_SHARED is dangerous, since it allows other agents to retarget the
> > IRQ's affinity without migrating PMU contexts to match, breaking the way
> > in which perf manages mutual exclusion for accessing events. Although
> > this means it's not realistically possible to support PMU IRQs being
> > shared with other drivers, we *can* handle sharing between multiple PMU
> > instances with some explicit affinity bookkeeping and manual interrupt
> > multiplexing.
> > 
> > RCU helps us handle interrupts efficiently without having to worry about
> > fine-grained locking for relatively-theoretical race conditions with the
> > probe/remove/CPU hotplug slow paths. The resulting machinery ends up
> > looking largely generic, so it should be feasible to factor out with a
> > "system PMU" base class for similar multi-instance drivers.
> > 
> > Signed-off-by: Robin Murphy <robin.murphy@....com>
> > ---
> > 
> > RFC because I don't have the means to test it, and if the general
> > approach passes muster then I'd want to tackle the aforementioned
> > factoring-out before merging anything anyway.
> 
> Any comments on whether it's worth pursuing this?

Sorry, I don't really get the problem that it's solving. Is there a crash
log somewhere I can look at? If all the users of the IRQ are managed by
this driver, why is IRQF_SHARED dangerous?

Will