Re: [PATCH 1 of 4] mm: add a ptep_modify_prot transaction abstraction

[Jeremy Fitzhardinge <jeremy@goop.org>]

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