Re: [PATCH v4 1/7] locking/pvqspinlock: Unconditional PV kick with _Q_SLOW_VAL

[Davidlohr Bueso <dave@stgolabs.net>]

On Mon, 2015-08-03 at 20:37 +0200, Peter Zijlstra wrote:
> OK, so there's no 'fix'? The patch claims we can loose a wakeup and I
> just don't see how that is true.

Taking another look, I think you could hit something like this:

CPU0 (lock):					CPU1 (unlock):
  pv_wait_head					  __pv_queued_spin_unlock
						    <load ->state> [bogus ->state != halted]
    <spin>					    smp_store_release(&l->locked, 0);
						    
    WRITE_ONCE(pn->state, vcpu_halted);             
    pv_wait(&l->locked, _Q_SLOW_VAL);		    if (->state == vcpu_halted)
					              pv_kick(node->cpu); <-- missing wakeup, never called

So basically you can miss a wakeup if node->state load is done while the
locking thread is spinning and hasn't gotten a chance to update the
state to halted. That would also imply that it occurs right when the
threshold limit is about to be reached.