Re: futex.c and fault handling

[Ingo Molnar <mingo@elte.hu>]

(extended the Cc: list with MM experts.)

* Darren Hart <dvhltc@us.ibm.com> wrote:

> I've been working in linux-tip core/futexes lately and have a need to be 
> able to properly handle faults for r/w access to a uaddr.  I was 
> planning on modeling this on the fault handling in futex_lock_pi which 
> used both get_user() and futex_handle_fault() to get the pages.  
> However, that used to be based on whether or not we held the mmap_sem.  
> Now that we're using fast_gup throughout futex.c, and the mmap_sem 
> locking has been pushed in tighter in get_futex_key(), I'm not sure if 
> the fault handling is still correct - the comments are certainly 
> incorrect since we no longer hold the mmap_sem when we hit 
> uaddr_faulted: inside futex_lock_pi (and a few other places have similar 
> comment vs. code dicrepancies):
>
> uaddr_faulted:
> 	/*
> 	 * We have to r/w  *(int __user *)uaddr, and we have to modify it
> 	 * atomically.  Therefore, if we continue to fault after get_user()
> 	 * below, we need to handle the fault ourselves, while still holding
> 	 * the mmap_sem.  This can occur if the uaddr is under contention as
> 	 * we have to drop the mmap_sem in order to call get_user().
> 	 */
> 	queue_unlock(&q, hb);
>
> 	if (attempt++) {
> 		ret = futex_handle_fault((unsigned long)uaddr, attempt);
> 		if (ret)
> 			goto out_put_key;
> 		goto retry_unlocked;
> 	}
>
> ---> previous versions dropped the mmap_sem here in preparation for get_user()
>
> 	ret = get_user(uval, uaddr);
> 	if (!ret)
> 		goto retry;
>
>
> So is the code still correct without the holding of mmap_sem?  I suppose 
> get_user() is still the more efficient path, and perhaps even more so 
> now that we don't have to release mmap_sem and reacquire it later in 
> order to call it.  If so, then I guess all that is needed is a comments 
> patch, which I'd be happy to write up.
>
> Thanks,
>
> -- 
> Darren Hart
> IBM Linux Technology Center
> Real-Time Linux Team