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From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 24 Mar 2017 12:27:16 -0700
Message-ID: <CALCETrVBJo7U9ZhamKn7h2MmhsFy6xOFYNU8PfE5KnXrkbAquw@mail.gmail.com>
Subject: Re: locking/atomic: Introduce atomic_try_cmpxchg()
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.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>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>,
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On Fri, Mar 24, 2017 at 12:20 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Fri, Mar 24, 2017 at 12:16 PM, Andy Lutomirski <luto@kernel.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.
>
> I doubt it's a show-stopper, if only because nobody cares about UP any
> more. Not even the embedded world does.
>
> That said, I'm not convinced that there will ever really be a reason
> for the kernel to use the C11 atomics. They just aren't any better
> than what we can do ourselves.
>
> The reason for C11 atomics is "portably good atomics". We use
> "architecture-specific good atomics" instead, and are willing to
> maintain that. We will *have* to maintain that in the forseeable
> future anyway, for legacy compiler issues.

In theory, though, the compiler could optimize based on its knowledge
of what the C11 atomics do.  ISTR reading about a few optimizations
that were already starting to be developed.

Using asm goto seems okay, too, but it's a lot more tedious is less
friendly to the optimizers.

--Andy