Date: Fri, 24 Mar 2017 17:41:08 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.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>
Subject: Re: locking/atomic: Introduce atomic_try_cmpxchg()
Message-ID: <20170324164108.ibcxxqbhvx6ao54r@hirez.programming.kicks-ass.net>
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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

Which is rather unfortunate...