Messages in this thread |  | | Date | Wed, 11 Apr 2018 16:22:12 +0100 | From | Catalin Marinas <> | Subject | Re: [PATCH 2/2] locking/qspinlock: Limit # of spins in _Q_PENDING_VAL wait loop |
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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
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