On Wed, 18 Jun 2008, Jeremy Fitzhardinge wrote:

It's not cmpxchg, just xchg. 
In other words, is:

	lock btr $_PAGE_BIT_RW, (%rbx)

Well, we can actually do it as

	lock andl $~_PAGE_RW,(%rbx)

although we haven't bothered (I've wanted several times to make 
clear_bit() do that, but have never gotten around to it - mainly because 
old gcc versions didn't work with __builtin_constant_p() in inline 
functions - so you have to do the macro from hell)

And yes, the "lock andl" should be noticeably faster than the xchgl.

			Linus