Re: [PATCH 8/9] qspinlock: Generic paravirt support

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Apr 01, 2015 at 07:12:23PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 01, 2015 at 12:20:30PM -0400, Waiman Long wrote:
> > After more careful reading, I think the assumption that the presence of an
> > unused bucket means there is no match is not true. Consider the scenario:
> > 
> > 1. cpu 0 puts lock1 into hb[0]
> > 2. cpu 1 puts lock2 into hb[1]
> > 3. cpu 2 clears hb[0]
> > 4. cpu 3 looks for lock2 and doesn't find it
> 
> Hmm, yes. The only way I can see that being true is if we assume entries
> are never taken out again.
> 
> The wikipedia page could use some clarification here, this is not clear.
> 
> > At this point, I am thinking using back your previous idea of passing the
> > queue head information down the queue.
> 
> Having to scan the entire array for a lookup sure sucks, but the wait
> loops involved in the other idea can get us in the exact predicament we
> were trying to get out, because their forward progress depends on other
> CPUs.
> 
> Hohumm.. time to think more I think ;-)

So bear with me, I've not really pondered this well so it could be full
of holes (again).

After the cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL) succeeds the
spin_unlock() must do the hash lookup, right? We can make the lookup
unhash.

If the cmpxchg() fails the unlock will not do the lookup and we must
unhash.