Messages in this thread | | | Date | Thu, 9 Apr 2015 21:57:21 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v15 13/15] pvqspinlock: Only kick CPU at unlock time |
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On Mon, Apr 06, 2015 at 10:55:48PM -0400, Waiman Long wrote:
> @@ -219,24 +236,30 @@ static void pv_wait_node(struct mcs_spinlock *node) > } > > /* > + * Called after setting next->locked = 1 & lock acquired. > + * Check if the the CPU has been halted. If so, set the _Q_SLOW_VAL flag > + * and put an entry into the lock hash table to be waken up at unlock time. > */ > -static void pv_kick_node(struct mcs_spinlock *node) > +static void pv_scan_next(struct qspinlock *lock, struct mcs_spinlock *node)
I'm not too sure about that name change..
> { > struct pv_node *pn = (struct pv_node *)node; > + struct __qspinlock *l = (void *)lock; > > /* > + * Transition CPU state: halted => hashed > + * Quit if the transition failed. > */ > + if (cmpxchg(&pn->state, vcpu_halted, vcpu_hashed) != vcpu_halted) > + return; > + > + /* > + * Put the lock into the hash table & set the _Q_SLOW_VAL in the lock. > + * As this is the same CPU that will check the _Q_SLOW_VAL value and > + * the hash table later on at unlock time, no atomic instruction is > + * needed. > + */ > + WRITE_ONCE(l->locked, _Q_SLOW_VAL); > + (void)pv_hash(lock, pn); > }
This is broken. The unlock path relies on:
pv_hash() MB l->locked = SLOW
such that when it observes SLOW, it must then also observe a consistent bucket.
The above can have us do pv_hash_find() _before_ we actually hash the lock, which will result in us triggering that BUG_ON() in there.
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