Re: arc_usr_cmpxchg and preemption

[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;
}