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

[Ingo Molnar]

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> 
> 
> 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).

for the oldfs stuff we already have probe_kernel_read(). OTOH, that 
involves pagefault_disable() which is an atomic op, so 
__get_user_careful() should be much more lightweight - and we already know 
that the memory range at least _used to_ be a valid kernel address.

(Theoretical race: with memory hotplug that kernel pointer address could 
have gotten unmapped and we could get device memory there - with 
side-effects if accessed. Wont happen in practice.)

	Ingo
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