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From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 24 Mar 2017 13:21:25 -0700
Message-ID: <CALCETrU=pXG5tgsFEvCvxmL5hM=qgywxQaqumm0_AzBb3KPopQ@mail.gmail.com>
Subject: Re: locking/atomic: Introduce atomic_try_cmpxchg()
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>, Dmitry Vyukov <dvyukov@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Borislav Petkov <bp@alien8.de>, Brian Gerst <brgerst@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Fri, Mar 24, 2017 at 1:14 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Fri, Mar 24, 2017 at 12:16:11PM -0700, Andy Lutomirski wrote:
>> On Fri, Mar 24, 2017 at 11:13 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
>> > Not to mention we cannot use the C11 atomics in kernel because we want
>> > to be able to runtime patch LOCK prefixes when only 1 CPU is available.
>>
>> Is this really a show-stopper?  I bet that objtool could be persuaded
>> to emit a list of the locations of all those LOCK prefixes.
>
> Ah, but its not _all_ LOCK prefixes. Some are needed even on UP, because
> against hardware instead of other CPUs. Or again hypervisor instead of
> other vCPU.
>

Make a table of mandatory lock prefixes and assume all the others (due
to C11 atomics, etc) can be omitted on UP?

I'm curious, though, whether anyone actually compiles an x86 SMP
kernel, runs it on UP, and cares about performance these days.

--Andy