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SubjectRe: [PATCH] x86: fix and improve cmpxchg_double{,_local}()
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Le lundi 02 janvier 2012 à 17:02 +0000, Jan Beulich a écrit :
> Just like the per-CPU ones they had several problems/shortcomings:
> Only the first memory operand was mentioned in the asm() operands, and
> the 2x64-bit version didn't have a memory clobber while the 2x32-bit
> one did. The former allowed the compiler to not recognize the need to
> re-load the data in case it had it cached in some register, while the
> latter was overly destructive.
>
> The types of the local copies of the old and new values were incorrect
> (the types of the pointed-to variables should be used here, to make
> sure the respective old/new variable types are compatible).
>
> The __dummy/__junk variables were pointless, given that local copies
> of the inputs already existed (and can hence be used for discarded
> outputs).
>
> The 32-bit variant of cmpxchg_double_local() referenced
> cmpxchg16b_local().
>
> At once also
> - change the return value type to what it really is: 'bool'
> - unify 32- and 64-bit variants
> - abstract out the common part of the 'normal' and 'local' variants
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

While looking at your patch, I discovered that atomic64_add() /
atomic64_inc() on 32bit are completely buggy. Oh well...

Generated code :

c03bc00c <atomic64_add_return_cx8>:
c03bc00c: 55 push %ebp
c03bc00d: 53 push %ebx
c03bc00e: 56 push %esi
c03bc00f: 57 push %edi
c03bc010: 89 c6 mov %eax,%esi
c03bc012: 89 d7 mov %edx,%edi
c03bc014: 89 cd mov %ecx,%ebp
c03bc016: 89 d8 mov %ebx,%eax
c03bc018: 89 ca mov %ecx,%edx
c03bc01a: f0 0f c7 4d 00 lock cmpxchg8b 0x0(%ebp)
c03bc01f: 89 c3 mov %eax,%ebx
c03bc021: 89 d1 mov %edx,%ecx
c03bc023: 01 f3 add %esi,%ebx
c03bc025: 11 f9 adc %edi,%ecx
c03bc027: f0 0f c7 4d 00 lock cmpxchg8b 0x0(%ebp)
c03bc02c: 75 f9 jne c03bc027 <atomic64_add_return_cx8+0x1b>
c03bc02e: 89 d8 mov %ebx,%eax
c03bc030: 89 ca mov %ecx,%edx
c03bc032: 5f pop %edi
c03bc033: 5e pop %esi
c03bc034: 5b pop %ebx
c03bc035: 5d pop %ebp
c03bc036: c3 ret

The ' jne c03bc027' should really be 'jne c03bc01f'

No idea how old is this bug.



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