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

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Date:   Fri, 24 Mar 2017 09:54:46 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.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 9:41 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Mar 24, 2017 at 03:21:40PM +0100, Peter Zijlstra wrote:
>> On Fri, Mar 24, 2017 at 01:44:00PM +0100, Dmitry Vyukov wrote:
>
>> > So I would suggest to change it to a safer and less surprising
>> > alternative:
>> >
>> > diff --git a/arch/x86/include/asm/cmpxchg.h b/arch/x86/include/asm/cmpxchg.h
>> > index fb961db51a2a..81fb985f51f4 100644
>> > --- a/arch/x86/include/asm/cmpxchg.h
>> > +++ b/arch/x86/include/asm/cmpxchg.h
>> > @@ -212,7 +212,8 @@ extern void __add_wrong_size(void)
>> >         default:                                                        \
>> >                 __cmpxchg_wrong_size();                                 \
>> >         }                                                               \
>> > -       *_old = __old;                                                  \
>> > +       if (!success)                                                   \
>> > +               *_old = __old;                                          \
>> >         success;                                                        \
>> >  })
>>
>> I've no immediate objection, I'll double check what, if anything, it
>> does for code gen.
>
> So the first snipped I tested regressed like so:
>
>
> 0000000000000000 <T_refcount_inc>:                              0000000000000000 <T_refcount_inc>:
>    0:   8b 07                   mov    (%rdi),%eax                 0:   8b 17                   mov    (%rdi),%edx
>    2:   83 f8 ff                cmp    $0xffffffff,%eax            2:   83 fa ff                cmp    $0xffffffff,%edx
>    5:   74 13                   je     1a <T_refcount_inc+0x1a>    5:   74 1a                   je     21 <T_refcount_inc+0x21>
>    7:   85 c0                   test   %eax,%eax                   7:   85 d2                   test   %edx,%edx
>    9:   74 0d                   je     18 <T_refcount_inc+0x18>    9:   74 13                   je     1e <T_refcount_inc+0x1e>
>    b:   8d 50 01                lea    0x1(%rax),%edx              b:   8d 4a 01                lea    0x1(%rdx),%ecx
>    e:   f0 0f b1 17             lock cmpxchg %edx,(%rdi)           e:   89 d0                   mov    %edx,%eax
>   12:   75 ee                   jne    2 <T_refcount_inc+0x2>     10:   f0 0f b1 0f             lock cmpxchg %ecx,(%rdi)
>   14:   ff c2                   inc    %edx                       14:   74 04                   je     1a <T_refcount_inc+0x1a>
>   16:   75 02                   jne    1a <T_refcount_inc+0x1a>   16:   89 c2                   mov    %eax,%edx
>   18:   0f 0b                   ud2                               18:   eb e8                   jmp    2 <T_refcount_inc+0x2>
>   1a:   c3                      retq                              1a:   ff c1                   inc    %ecx
>                                                                   1c:   75 03                   jne    21 <T_refcount_inc+0x21>
>                                                                   1e:   0f 0b                   ud2
>                                                                   20:   c3                      retq
>                                                                   21:   c3                      retq

Can you re-send the better asm you got earlier?

If I pretend to be a dumb compiler, I wonder if you'd get better results with:

if (!success) {
  *_old = __old;
  return false;
} else {
  return true;
}

or however you jam that into a statement expression.  That way you
aren't relying on the compiler to merge the branches.

>
> Which is rather unfortunate...



-- 
Andy Lutomirski
AMA Capital Management, LLC