arc_usr_cmpxchg and preemption

[peterz@xxxxxxxxxxxxx (Peter Zijlstra)]

On Wed, Mar 14, 2018 at 09:58:19AM -0700, Vineet Gupta wrote:

> Well it is broken wrt the semantics the syscall is supposed to provide.
> Preemption disabling is what prevents a concurrent thread from coming in and
> modifying the same location (Imagine a variable which is being cmpxchg
> concurrently by 2 threads).
> 
> One approach is to do it the MIPS way, emulate the llsc flag - set it under
> preemption disabled section and clear it in switch_to

*shudder*... just catch the -EFAULT, force the write fault and retry.

Something like:

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;
	}

	return ret;
}