linux-kernel - Re: locking/atomic: Introduce atomic_try

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Message-ID: <20170324201439.GY5680@worktop>
Date:   Fri, 24 Mar 2017 21:14:39 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: locking/atomic: Introduce atomic_try_cmpxchg()

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@...radead.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.