Hi Peter, Vineet,
On Wed, 2018-03-14 at 18:53 +0100, Peter Zijlstra wrote:
> 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;
I guess this jump we need to do only once, right?
If for whatever reason get_user_pages_fast() fails we return immediately
and if it succeeds there's no reason for put_user() to not succeed as
required page is supposed to be prepared for write.
Otherwise if something goes way too bad we may end-up in an infinite loop
which we'd better prevent.
> }
>
> return ret;
> }
@Vineet, are you OK with proposed implementation?
-Alexey
