Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning

[Linus Torvalds <torvalds@linux-foundation.org>]

On Wed, 7 Jan 2009, Matthew Wilcox wrote:
> 
> I appreciate this is sample code, but using __get_user() on
> non-userspace pointers messes up architectures which have separate
> user/kernel spaces (eg the old 4G/4G split for x86-32).  Do we have an
> appropriate function for kernel space pointers?  Is this a good reason
> to add one?

Yes, you're right. 

We could do the whole "oldfs = get_fs(); set_fs(KERNEL_DS); .. 
set_fs(oldfs);" crud, but it would probably be better to just add an 
architected accessor. Especially since it's going to generally just be a

	#define get_kernel_careful(val,p) __get_user(val,p)

for most architectures.

We've needed that before (and yes, we've simply mis-used __get_user() on 
x86 before rather than add it).

			Linus