So the reason I never get around to this is because the patch stinks.
It simply doesn't make sense... Remember, the harder you make a reviewer
work the less likely the review will be done.
Present things in clear concise language and draw a picture.
On Tue, May 31, 2016 at 12:53:48PM -0400, Waiman Long wrote:
> Currently, calling pv_hash() and setting _Q_SLOW_VAL is only
> done once for any pv_node. It is either in pv_kick_node() or in
> pv_wait_head_or_lock().
So far so good....
> Because of lock stealing, a pv_kick'ed node is
> not guaranteed to get the lock before the spinning threshold expires
> and has to call pv_wait() again. As a result, the new lock holder
> won't see _Q_SLOW_VAL and so won't wake up the sleeping vCPU.
*brain melts* what!? pv_kick'ed node reads like pv_kick_node() and that
doesn't make any kind of sense.
I'm thinking you're trying to say this:
CPU0 CPU1 CPU2
__pv_queued_spin_unlock_slowpath()
...
smp_store_release(&l->locked, 0);
__pv_queued_spin_lock_slowpath()
...
pv_queued_spin_steal_lock()
cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0
pv_wait_head_or_lock()
pv_kick(node->cpu); ----------------------> pv_wait(&l->locked,
_Q_SLOW_VAL);
__pv_queued_spin_unlock()
cmpxchg(&l->locked, _Q_LOCKED_VAL, 0) == _Q_LOCKED_VAL
for () {
trylock_clear_pending();
cpu_relax();
}
pv_wait(&l->locked,
_Q_SLOW_VAL);
Which is indeed 'bad', but not fatal, note that the later pv_wait() will
not in fact go wait, since l->locked will _not_ be _Q_SLOW_VAL.
Is this indeed the 3 CPU scenario you tried to describe in a scant 4
lines of text, or is there more to it?
