[PATCH v2] kvm: fix waitqueue_active without memory barrier in virt/kvm/async_pf.c

[Kosuke Tatsukawa]

async_pf_execute() seems to be missing a memory barrier which might
cause the waker to not notice the waiter and miss sending a wake_up as
in the following figure.

        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)
                                     ...
                                     return 0;
list_add_tail(&apf->link,
  &vcpu->async_pf.done);
spin_unlock(&vcpu->async_pf.lock);
                                    waited = true;
                                    schedule();
------------------------------------------------------------------------

The attached patch adds the missing memory barrier.

I found this issue when I was looking through the linux source code
for places calling waitqueue_active() before wake_up*(), but without
preceding memory barriers, after sending a patch to fix a similar
issue in drivers/tty/n_tty.c  (Details about the original issue can be
found here: https://lkml.org/lkml/2015/9/28/849).

Signed-off-by: Kosuke Tatsukawa <tatsu@xxxxxxxxxxxxx>
---
v2:
  - Fixed comment based on feedback from Paolo
v1:
  - https://lkml.org/lkml/2015/10/8/994
---
 virt/kvm/async_pf.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
index 44660ae..a0999d7 100644
--- a/virt/kvm/async_pf.c
+++ b/virt/kvm/async_pf.c
@@ -94,6 +94,12 @@ static void async_pf_execute(struct work_struct *work)
 
 	trace_kvm_async_pf_completed(addr, gva);
 
+	/*
+	 * Memory barrier is required here to make sure change to
+	 * vcpu->async_pf.done is visible from other CPUs.  This memory
+	 * barrier pairs with prepare_to_wait's set_current_state()
+	 */
+	smp_mb();
 	if (waitqueue_active(&vcpu->wq))
 		wake_up_interruptible(&vcpu->wq);
 
-- 
1.7.1
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