Linus Torvalds wrote:
> On Wed, 18 Jun 2008, Jeremy Fitzhardinge wrote:
>
>> Along the lines of:
>>
>
> Hell no. There's a reason we have a special set_wrprotect() thing. We can
> do it more efficiently on native hardware by just clearing the bit
> atomically. No need to do the cmpxchg games.
>
It's not cmpxchg, just xchg.
In other words, is:
lock btr $_PAGE_BIT_RW, (%rbx)
much cheaper than
mov $0, %rax
xchg %rax, (%rbx)
and $~_PAGE_RW, %rax
mov %rax, (%rbx)
?
It's the same number of locked RMW operations, so aside from being a few
instructions longer, I think it would be much the same.
I guess the correct answer is "lmbench".
J
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