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...
