Re: locking/atomic: Introduce atomic_try_cmpxchg()

[Peter Zijlstra]

On Fri, Mar 24, 2017 at 07:08:38PM +0100, Peter Zijlstra wrote:
> On Fri, Mar 24, 2017 at 06:51:15PM +0100, Dmitry Vyukov wrote:
> > On Fri, Mar 24, 2017 at 6:23 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > On Fri, Mar 24, 2017 at 09:54:46AM -0700, Andy Lutomirski wrote:
> > >> > 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
> > >>
> 
> > This seems to help ;)
> > 
> > #define try_cmpxchg(ptr, pold, new) __atomic_compare_exchange_n(ptr, pold, new, 0, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST)
> 
> That gets me:
> 
> 0000000000000000 <T_refcount_inc>:
>    0:   8b 07                   mov    (%rdi),%eax
>    2:   89 44 24 fc             mov    %eax,-0x4(%rsp)
>    6:   8b 44 24 fc             mov    -0x4(%rsp),%eax
>    a:   83 f8 ff                cmp    $0xffffffff,%eax
>    d:   74 1c                   je     2b <T_refcount_inc+0x2b>
>    f:   85 c0                   test   %eax,%eax
>   11:   75 07                   jne    1a <T_refcount_inc+0x1a>
>   13:   8b 44 24 fc             mov    -0x4(%rsp),%eax
>   17:   0f 0b                   ud2    
>   19:   c3                      retq   
>   1a:   8d 50 01                lea    0x1(%rax),%edx
>   1d:   8b 44 24 fc             mov    -0x4(%rsp),%eax
>   21:   f0 0f b1 17             lock cmpxchg %edx,(%rdi)
>   25:   75 db                   jne    2 <T_refcount_inc+0x2>
>   27:   ff c2                   inc    %edx
>   29:   74 e8                   je     13 <T_refcount_inc+0x13>
>   2b:   c3                      retq  
> 
> 
> Which is even worse... (I did double check it actually compiled)

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.