On Tue, Aug 09, 2016 at 05:37:47PM +0800, Wanpeng Li wrote: > From: Wanpeng Li <wanpeng...@hotmail.com> > > When the lock holder vCPU is racing with the queue head vCPU: > > lock holder vCPU queue head vCPU > ===================== ================== > > node->locked = 1; > <preemption> READ_ONCE(node->locked) > ... pv_wait_head_or_lock(): > SPIN_THRESHOLD loop; > pv_hash(); > lock->locked = _Q_SLOW_VAL; > node->state = vcpu_hashed; > pv_kick_node(): > cmpxchg(node->state, > vcpu_halted, vcpu_hashed); > lock->locked = _Q_SLOW_VAL; > pv_hash();
So here the example is 'wrong' in that it doesn't illustrate the fail case, namely having vcpu_halted win while we're hashed. > +++ b/kernel/locking/qspinlock_paravirt.h > @@ -450,7 +450,28 @@ pv_wait_head_or_lock(struct qspinlock *lock, struct > mcs_spinlock *node) > goto gotlock; > } > } > - WRITE_ONCE(pn->state, vcpu_halted); > + /* > + * lock holder vCPU queue head vCPU > + * ---------------- --------------- > + * node->locked = 1; > + * <preemption> READ_ONCE(node->locked) > + * ... pv_wait_head_or_lock(): > + * SPIN_THRESHOLD loop; > + * pv_hash(); > + * lock->locked = _Q_SLOW_VAL; > + * node->state = vcpu_hashed; > + * pv_kick_node(): > + * cmpxchg(node->state, > + * vcpu_halted, vcpu_hashed); > + * lock->locked = _Q_SLOW_VAL; > + * pv_hash(); > + * > + * With preemption at the right moment, it is possible that > both the > + * lock holder and queue head vCPUs can be racing to set > node->state. > + * Making sure the state is never set to vcpu_halted will > prevent this > + * racing from happening. > + */ > + WRITE_ONCE(pn->state, vcpu_hashed); > qstat_inc(qstat_pv_wait_head, true); > qstat_inc(qstat_pv_wait_again, waitcnt); > pv_wait(&l->locked, _Q_SLOW_VAL); And I completely fail to see the point of this comment, still. Yes, if we would have used vcpu_halted, there would be a problem, but we don't so there isn't. What does this comment tell us about the current code? In any case, I have an older version (possibly v1) queued up. That fixes the bug.
