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Date: Tue, 03 Jan 2012 16:00:30 +0100 From: Eric Dumazet <eric.dumazet@...il.com> To: Jan Beulich <JBeulich@...e.com> Cc: mingo@...e.hu, tglx@...utronix.de, hpa@...or.com, Christoph Lameter <cl@...ux.com>, linux-kernel@...r.kernel.org, netdev <netdev@...r.kernel.org> Subject: Re: [PATCH] x86: fix and improve cmpxchg_double{,_local}() 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@...e.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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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