Re: [PATCH v2 05/11] locking/ww_mutex: Add waiters in stamp order

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Dec 16, 2016 at 03:19:43PM +0100, Nicolai Hähnle wrote:
> Hi Peter and Chris,
> 
> (trying to combine the handoff discussion here)
> 
> On 06.12.2016 17:55, Peter Zijlstra wrote:
> >On Thu, Dec 01, 2016 at 03:06:48PM +0100, Nicolai Hähnle wrote:
> >>@@ -693,8 +748,12 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> >> 		 * mutex_unlock() handing the lock off to us, do a trylock
> >> 		 * before testing the error conditions to make sure we pick up
> >> 		 * the handoff.
> >>+		 *
> >>+		 * For w/w locks, we always need to do this even if we're not
> >>+		 * currently the first waiter, because we may have been the
> >>+		 * first waiter during the unlock.
> >> 		 */
> >>-		if (__mutex_trylock(lock, first))
> >>+		if (__mutex_trylock(lock, use_ww_ctx || first))
> >> 			goto acquired;
> >
> >So I'm somewhat uncomfortable with this. The point is that with the
> >.handoff logic it is very easy to accidentally allow:
> >
> >	mutex_lock(&a);
> >	mutex_lock(&a);
> >
> >And I'm not sure this doesn't make that happen for ww_mutexes. We get to
> >this __mutex_trylock() without first having blocked.
> 
> Okay, took me a while, but I see the problem. If we have:
> 
> 	ww_mutex_lock(&a, NULL);
> 	ww_mutex_lock(&a, ctx);
> 
> then it's possible that another currently waiting task sets the HANDOFF flag
> between those calls and we'll allow the second ww_mutex_lock to go through.

Its worse, __mutex_trylock() doesn't check if MUTEX_FLAG_HANDOFF is set,
if .handoff == true && __owner_task() == current, we 'acquire'.

And since 'use_ww_ctx' is unconditionally true for ww_mutex_lock(), the
sequence:

	ww_mutex_lock(&a, ...);
	ww_mutex_lock(&a, ...);

will 'work'.