linux-kernel - Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless update of refcount

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Date:	Tue, 3 Sep 2013 14:39:42 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Sedat Dilek <sedat.dilek@...il.com>,
	Waiman Long <waiman.long@...com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Jeff Layton <jlayton@...hat.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...radead.org>
Subject: Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless
 update of refcount

On Tue, Sep 3, 2013 at 2:34 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> I'll try to hack that up too, but it's looking like it really is just
> the "lock xadd", not the memory dependency chain..

Yeah, no difference: Better code generation with my quick hack for a
percpu spinlock:

       │    ffffffff81078e70 <lg_local_lock>:
  0.59 │      push   %rbp
  0.25 │      mov    %rsp,%rbp
  0.07 │      mov    $0x100,%eax
 97.55 │      lock   xadd   %ax,%gs:(%rdi)
  0.01 │      movzbl %ah,%edx
       │      cmp    %al,%dl
  0.68 │    ↓ je     29
       │      nop
       │20:   pause
       │      mov    %gs:(%rdi),%al
       │      cmp    %dl,%al
       │    ↑ jne    20
       │29:   pop    %rbp
  0.84 │    ← retq

but the actual cost is pretty much the same:

   6.81%   lg_local_lock

so it doesn't seem to be some odd weakness of the microarchitecture.

            Linus
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