Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20190227101252.413192716@infradead.org> <20190227101544.210748832@infradead.org>
In-Reply-To: <20190227101544.210748832@infradead.org>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 27 Feb 2019 08:14:09 -0800
Message-ID: <CAHk-=wiv6dgJDj81wukpJCJBVhAVn-CYJdhj+o8JGN1LfRwb0g@mail.gmail.com>
Subject: Re: [PATCH 1/5] x86/percpu: Differentiate this_cpu_{}() and __this_cpu_{}()
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Lutomirski <luto@kernel.org>,
        Nadav Amit <namit@vmware.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Nadav Amit <nadav.amit@gmail.com>
Content-Type: multipart/mixed; boundary="000000000000b6842c0582e277a3"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

--000000000000b6842c0582e277a3
Content-Type: text/plain; charset="UTF-8"

On Wed, Feb 27, 2019 at 2:16 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> Nadav Amit reported that commit:
>
>   b59167ac7baf ("x86/percpu: Fix this_cpu_read()")
>
> added a bunch of constraints to all sorts of code; and while some of
> that was correct and desired, some of that seems superfluous.

Hmm.

I have the strong feeling that we should instead relax this_cpu_read()
again a bit.

In particular, making it "asm volatile" really is a big hammer
approach. It's worth noting that the *other* this_cpu_xyz ops don't
even do that.

I would suggest that instead of making "this_cpu_read()" be asm
volatile, we mark it as potentially changing the memory location it is
touching - the same way the modify/write ops do.

That still means that the read will be forced (like READ_ONCE()), but
allows gcc a bit more flexibility in instruction scheduling, I think.

Trivial (but entirely untested) patch attached.

That said, I didn't actually check how it affects code generation.
Nadav, would you check the code sequences you originally noticed?

                   Linus

--000000000000b6842c0582e277a3
Content-Type: text/x-patch; charset="US-ASCII"; name="patch.diff"
Content-Disposition: attachment; filename="patch.diff"
Content-Transfer-Encoding: base64
Content-ID: <f_jsnegv090>
X-Attachment-Id: f_jsnegv090
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--000000000000b6842c0582e277a3--