Hi Paolo,
Looking at the history of this function, is it reasonable to say it
fixes the following commit?
af585b9 KVM: Halt vcpu if page it tries to access is swapped out
Does it make it a good candidate for -stable?
Thanks,
On Oct09 12:21, Kosuke Tatsukawa wrote:
> 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
--
William
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