On Thu, Apr 09, 2015 at 09:57:21PM +0200, Peter Zijlstra wrote:
> 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.
Urgh, clearly its late and I cannot read. The comment explains it.
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