linux-kernel - Re: [PATCH v4] locking/lock_events: Use this_cpu

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Date:   Tue, 28 May 2019 10:22:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Waiman Long <longman@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH v4] locking/lock_events: Use this_cpu_add() when necessary

On Mon, May 27, 2019 at 12:33:56PM -0700, Linus Torvalds wrote:
> On Mon, May 27, 2019 at 1:23 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > That's disguisting... I see Linus already applied it, but yuck. That's
> > what we have raw_cpu_*() for.
> 
> Ahh, I tried to look for that, but there was enough indirection and
> confusion that I wasn't sure they were generically available.
> 
> And the "raw_cpu_*()" functions are rare enough that I'd never
> encountered them enough to really be aware of them. In fact, we seem
> to have exactly _one_ user of "raw_cpu_add()" in the whole kernel, and
> a handful of "raw_cpu_inc()".

Yeah, not having many is good. From a correctness PoV they're basically
always the wrong thing to use, except for this one usecase where we
prefer speed over correctness.

> But ack on your patch, and a heartfelt "yeah, that's the right thing". Thanks,

Thanks, I'll go write me a Changelog then ;-)