Re: [PATCH] kvm: fix waitqueue_active without memory barrier in virt/kvm/async_pf.c

[Paolo Bonzini]

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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