Hi Waiman,
On Mon, Apr 09, 2018 at 02:08:52PM -0400, Waiman Long wrote:
> @@ -311,13 +320,19 @@ void queued_spin_lock_slowpath(struct qspinlock *lock,
> u32 val)
> return;
>
> /*
> - * wait for in-progress pending->locked hand-overs
> + * wait for in-progress pending->locked hand-overs with a
> + * limited number of spins.
> *
> * 0,1,0 -> 0,0,1
> */
> if (val == _Q_PENDING_VAL) {
> - while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL)
> + int cnt = _Q_PENDING_LOOP;
> +
> + while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL) {
> + if (!--cnt)
> + goto queue;
> cpu_relax();
> + }
> }
In my model, the pathological case is not this loop but the following
one (trylock || pending):
P0: P1:
queued_spin_lock() fails queued_spin_lock() succeeds
queued_spin_lock_slowpath()
val == _Q_LOCKED_VAL
new = _Q_LOCKED_VAL |
_Q_PENDING_VAL
queued_spin_unlock()
lock->val == 0
cmpxchg(&lock->val, val, new)
fails
val = old (0)
repeat for (;;) loop:
new = _Q_LOCKED_VAL
queued_spin_lock() succeeds
lock->val == _Q_LOCKED_VAL
cmpxchg(&lock->val, val, new)
fails
val = old (_Q_LOCKED_VAL)
repeat for (;;) loop:
... and we are back to the P0 state above when it first entered
the loop, hence no progress. P1 never enters slowpath.
I think the pending bounded loop in your patch is needed for a three CPU
scenario where two of them can hand over _Q_PENDING_VAL while the third
doesn't make progress. I tried modeling 3 CPUs to see but the tool still
hits the for (;;) loop case rather than pending wait loop. Maybe a
combination of Will's changes to the (trylock || pending) loop with your
bounded pending hand-over?
--
Catalin
