Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.31 as permitted sender) client-ip=23.128.96.31;
Date:   Thu, 19 Oct 2023 10:54:32 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Uros Bizjak <ubizjak@gmail.com>, Nadav Amit <namit@vmware.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()
Message-ID: <20231019085432.GQ33217@noisy.programming.kicks-ass.net>
References: <CAFULd4Yy-v40tK94rexSOL99FGMke2Jk42wgcjoEBxV=2hXoCw@mail.gmail.com>
 <CAHk-=wjrLoy6xEDXB=piEUagDLMmV5Up7UK75W1D0E0UFVO-iA@mail.gmail.com>
 <CAFULd4autFT=96EckL9vUDgO5t0ESp27+NDVXQHGi7N=PAo-HQ@mail.gmail.com>
 <CAFULd4Zhw=zoDtir03FdPxJD15GZ5N=SV9=4Z45_Q_P9BL1rvQ@mail.gmail.com>
 <CAHk-=wgoWOcToLYbuL2GccbNXwj_MH-LxmB_7MMjw6uu50k57Q@mail.gmail.com>
 <CAHk-=wgCPbkf0Kdi=4T3LAVvNEH0jxJBWcTiBkrFDBsxkC9mKQ@mail.gmail.com>
 <CAFULd4aTY002A7NHRCX21aTpYOE=tnpouBk6hkoeWND=LnT4ww@mail.gmail.com>
 <CAHk-=wia9vFmyCJPkYg0vvegF8eojLy+DxVtpfoDv-UHoWKfqQ@mail.gmail.com>
 <CAFULd4Zj5hTvATZUVYhUGrxH3fiAUWjO9C27UV_USf2H164thQ@mail.gmail.com>
 <CAHk-=whEc2HR3En32uyAufPM3tEh8J4+dot6JyGW=Eg5SEhx7A@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAHk-=whEc2HR3En32uyAufPM3tEh8J4+dot6JyGW=Eg5SEhx7A@mail.gmail.com>
Precedence: bulk

On Wed, Oct 18, 2023 at 03:40:05PM -0700, Linus Torvalds wrote:

> Look at the *REAL* sequence counter code in <linux/seqlock.h>. Notice
> how in raw_read_seqcount_begin() we have
> 
>         unsigned _seq = __read_seqcount_begin(s);
>         smp_rmb();
> 
> because it actually does the proper barriers. Notice how the garbage
> code in __cyc2ns_read() doesn't have them - and how it was buggy as a
> result.
> 
> (Also notice how this all predates our "we should use load_acquire()
> instead of smb_rmb()", but whatever).

seqlock actually wants rmb even today, the pattern is:

    do {
	seq = load-seq
	rmb
	load-data
	rmb
    } while (seq != re-load-seq)

we specifically only care about loads, and the data loads must be
between the sequence number loads.

As such, load-acquire is not a natural match.