From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx> 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(); 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 which can result in hash entry race. Making sure the state is never set to vcpu_halted will prevent this racing from happening. This patch fix it by setting vcpu_hashed after we did all hash thing. Acked-by: Waiman Long <Waiman.Long@xxxxxxx> Reviewed-by: Davidlohr Bueso <dave@xxxxxxxxxxxx> Reviewed-by: Pan Xinhui <xinhui.pan@xxxxxxxxxxxxxxxxxx> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Waiman Long <Waiman.Long@xxxxxxx> Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx> --- v3 -> v4: * update patch subject * add code comments v2 -> v3: * fix typo in patch description v1 -> v2: * adjust patch description kernel/locking/qspinlock_paravirt.h | 23 ++++++++++++++++++++++- 1 file changed, 22 insertions(+), 1 deletion(-) diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h index 21ede57..ca96db4 100644 --- a/kernel/locking/qspinlock_paravirt.h +++ 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); -- 2.1.0 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html