arc_usr_cmpxchg and preemption

[Peter Zijlstra]

On Wed, Mar 14, 2018 at 08:38:53PM +0000, Alexey Brodkin wrote:
> > int sys_cmpxchg(u32 __user *user_ptr, u32 old, u32 new)
> > {
> > 	u32 val;
> > 	int ret;
> > 
> > again:
> > 	ret = 0;
> > 
> > 	preempt_disable();
> > 	val = get_user(user_ptr);
> > 	if (val == old)
> > 		ret = put_user(new, user_ptr);
> > 	preempt_enable();
> > 
> > 	if (ret == -EFAULT) {
> > 		struct page *page;
> > 		ret = get_user_pages_fast((unsigned long)user_ptr, 1, 1, &page);
> > 		if (ret < 0)
> > 			return ret;
> > 		put_page(page);
> > 		goto again;
> 
> I guess this jump we need to do only once, right?

Typically, yes. It is theoretically possible for the page to get
paged-out right after we do put_page() and before we do get/put_user(),
But if that happens the machine likely has bigger problems than having
to do this loop again.

FWIW, look at kernel/futex.c for working examples of this pattern, the
above was written purely from memory and could contain a fail or two ;-)

Also, it might make sense to stuff this implementation in some lib/ file
somewhere and make all platforms that need it use the same code, afaict
there really isn't anything platform specific to it.