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.
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