linux-kernel - Re: [PATCH 2/4] soc: qcom: icc-bwmon: Allow for interrupts to be shared across instances

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Message-ID: <r6bwmhfa4csubsvetnjlj6gzgovewupxf6hkuygqdconldpk2v@otrs4lhd3baj>
Date: Sat, 15 Jun 2024 00:42:15 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Sibi Sankar <quic_sibis@...cinc.com>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>, 
	andersson@...nel.org, konrad.dybcio@...aro.org, djakov@...nel.org, robh+dt@...nel.org, 
	krzysztof.kozlowski+dt@...aro.org, srinivas.kandagatla@...aro.org, linux-kernel@...r.kernel.org, 
	linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org, linux-pm@...r.kernel.org, 
	quic_rgottimu@...cinc.com, quic_kshivnan@...cinc.com, conor+dt@...nel.org, 
	abel.vesa@...aro.org
Subject: Re: [PATCH 2/4] soc: qcom: icc-bwmon: Allow for interrupts to be
 shared across instances

On Sat, Jun 15, 2024 at 01:49:34AM GMT, Sibi Sankar wrote:
> 
> 
> On 6/14/24 13:54, Krzysztof Kozlowski wrote:
> > On 13/06/2024 19:02, Sibi Sankar wrote:
> > > 
> > > 
> > > On 6/4/24 12:16, Krzysztof Kozlowski wrote:
> > > > On 04/06/2024 03:11, Sibi Sankar wrote:
> > > > > The multiple BWMONv4 instances available on the X1E80100 SoC use the
> > > > > same interrupt number. Mark them are shared to allow for re-use across
> > > > > instances.
> > > 
> > > Hey Krzysztof,
> > > 
> > > Thanks for taking time to review the series :)
> > > 
> > > > 
> > > > Would be nice if you also mention you checked that it is safe to have
> > > > both devm and shared interrupts (so you investigated possibility of race
> > > > on exit path).
> > > 
> > > I didn't see any problems with devm being used with SHARED when I posted
> > > it out. After your review comments I went back again to vett the exit
> > > path for races and ran into an pre-existing splat [1] but the bwmon
> > > instances work as expected on module removal/re-insertion.
> > 
> > Using devm and shared interrupts is in general sign of possible race
> > issues and should be avoided. Just "not seeing problems" is not an
> > argument for me, to be honest.
> 
> Didn't I go further and say I got it tested though? Also can you
> elaborate on what race do you think the bwmon will hit rather than
> being too generic about it?

devm_request_threaded_irq means that the IRQ is freed after the
bwmon_remove() function returns. Having IRQF_SHARED means that the IRQ
can still be triggered even though IRQ for this device has been disabled
in bwmon_disable().

In this particular case such IRQ probably won't cause issues, but at
least it needs to be validated and probably commented in bwmon_remove().
Just stating that "you tested and had no problems" usually isn't enough
for the expected race condition issues.

-- 
With best wishes
Dmitry