linux-kernel - Re: [PATCH v4 7/8] lockdep: Change hardirq{s_enabled,

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Message-ID: <20200624123147.GH4781@hirez.programming.kicks-ass.net>
Date:   Wed, 24 Jun 2020 14:31:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marco Elver <elver@...gle.com>
Cc:     "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>, bigeasy@...utronix.de,
        "David S. Miller" <davem@...emloft.net>,
        sparclinux@...r.kernel.org, Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, heiko.carstens@...ibm.com,
        linux-s390@...r.kernel.org, linux@...linux.org.uk,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v4 7/8] lockdep: Change hardirq{s_enabled,_context} to
 per-cpu variables

On Wed, Jun 24, 2020 at 12:17:56PM +0200, Marco Elver wrote:
> On Wed, 24 Jun 2020 at 11:01, Peter Zijlstra <peterz@...radead.org> wrote:

> > And I figured a quick way to get rid of that would be something like the
> > below, seeing how volatile gets auto annotated... but that doesn't seem
> > to actually work.
> >
> > What am I missing?
> 
> There's one more in include/linux/rcupdate.h. I suggested this at some point:
> 
>     https://lore.kernel.org/lkml/20200220213317.GA35033@google.com/
> 
> To avoid volatiles as I don't think they are needed here.

Urgghh.. local_t is very expensive for this. The current code is
actually fine, even on load-store architectures. Using local_t will only
result in it being more expensive for no gain.

I'll go put data_race() around it.