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

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Message-ID: <20230616090637.GB4253@hirez.programming.kicks-ass.net>
Date:   Fri, 16 Jun 2023 11:06:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marc Zyngier <maz@...nel.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, Jun 12, 2023 at 04:03:24PM +0100, Marc Zyngier wrote:
> 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.

Can I get a Fixes tag?