Re: [PATCH] x86: fix and improve cmpxchg_double{,_local}()

Le mardi 03 janvier 2012 à 16:00 +0100, Eric Dumazet a écrit :

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.

Very old it seems...

arch/x86/lib/atomic64_cx8_32.S

all "jxx 1b" are wrong if a LOCK_PREFIX is included after the 1: label

1:
	inst1
	LOCK_PREFIX
	cmpxchg8b (%ebp)
	jne 1b  / jumps to beginning of LOCK_PREFIX, inst1 is not replayed