On 09/10/2015 02:35, Kosuke Tatsukawa wrote:
> async_pf_execute kvm_vcpu_block
> ------------------------------------------------------------------------
> spin_lock(&vcpu->async_pf.lock);
> if (waitqueue_active(&vcpu->wq))
> /* The CPU might reorder the test for
> the waitqueue up here, before
> prior writes complete */
> prepare_to_wait(&vcpu->wq, &wait,
> TASK_INTERRUPTIBLE);
> /*if (kvm_vcpu_check_block(vcpu) < 0) */
> /*if (kvm_arch_vcpu_runnable(vcpu)) { */
> ...
> return (vcpu->arch.mp_state == KVM_MP_STATE_RUNNABLE &&
> !vcpu->arch.apf.halted)
> || !list_empty_careful(&vcpu->async_pf.done)
> ...
The new memory barrier isn't "paired" with any other, and in
fact I think that the same issue exists on the other side:
list_empty_careful(&vcpu->async_pf.done) may be reordered up,
before the prepare_to_wait:
spin_lock(&vcpu->async_pf.lock);
(vcpu->arch.mp_state == KVM_MP_STATE_RUNNABLE &&
!vcpu->arch.apf.halted)
|| !list_empty_careful(&vcpu->async_pf.done)
...
prepare_to_wait(&vcpu->wq, &wait,
TASK_INTERRUPTIBLE);
/*if (kvm_vcpu_check_block(vcpu) < 0) */
/*if (kvm_arch_vcpu_runnable(vcpu)) { */
...
return 0;
list_add_tail(&apf->link,
&vcpu->async_pf.done);
spin_unlock(&vcpu->async_pf.lock);
waited = true;
schedule();
if (waitqueue_active(&vcpu->wq))
So you need another smp_mb() after prepare_to_wait(). I'm not sure
if it's needed also for your original tty report, but I think it is
for https://lkml.org/lkml/2015/10/8/989 ("mei: fix waitqueue_active
without memory barrier in mei drivers").
I wonder if it makes sense to introduce smp_mb__before_spin_lock()
and smp_mb__after_spin_unlock(). On x86 the former could be a
simple compiler barrier, and on s390 both of them could. But that
should be a separate patch.
Thanks,
Paolo
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