linux-kernel - Re: [PATCH] perf/core: Drop __weak attribute from arch_perf_update

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Message-ID: <86r0qgbu4j.wl-maz@kernel.org>
Date:   Mon, 12 Jun 2023 16:03:24 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org,
        Reiji Watanabe <reijiw@...gle.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH] perf/core: Drop __weak attribute from arch_perf_update_userpage() prototype

On Mon, 12 Jun 2023 15:54:23 +0100,
Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Mon, Jun 12, 2023 at 02:16:28PM +0100, Mark Rutland wrote:
> > On Sat, Jun 03, 2023 at 09:25:19AM +0100, Marc Zyngier wrote:
> > > Reiji reports that the arm64 implementation of arch_perf_update_userpage()
> > > is now ignored and replaced by the dummy stub in core code.
> > > This seems to happen since the PMUv3 driver was moved to driver/perf.
> > 
> > I guess we should have a Cc stable then?
> > 
> > The below implies this has always been on dodgy ground, and so it's probably
> > inaccurate to give this a Fixes tag pointing to the move.
> > 
> > > As it turns out, dropping the __weak attribute from the *prototype*
> > > of the function solves the problem. You're right, this doesn't seem
> > > to make much sense. And yet... It appears that both symbols get
> > > flagged as weak, and that the first one to appear in the link order
> > > wins:
> > > 
> > > $ nm drivers/perf/arm_pmuv3.o|grep arch_perf_update_userpage
> > > 0000000000001db0 W arch_perf_update_userpage
> > 
> > Ah, so having it on th *declaration* will apply to any *definition*. :/
> 
> Yikes..
> 
> > That suggests this is a bad pattern generally, and we should probably remove
> > the other __weak instances in headers. Lukcily it seems there aren't that many:
> > 
> > [mark@...rids:~/src/linux]% git grep __weak -- **/*.h | wc -l
> > 50
> > 
> > IMO we'd should aim to remove __weak entirely; it causes a number of weird
> > things like this and it'd be much easier to manage with a small amount of
> > ifdeffery.
> > 
> > Peter, thoughts?
> 
> Not a fan of __weak myself, after having had to deal with how the
> compilers actually make it work.
> 
> Where do I queue this? perf/urgent?

That'd be my preference, as arm64 is currently a bit broken and I'd
like 6.4 to be functional.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.