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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