On 30/11/2018 13:32, Michael S. Tsirkin wrote:
> On Fri, Nov 30, 2018 at 11:37:02AM +0000, Jean-Philippe Brucker wrote:
>> Commit 78139c94dc8c ("net: vhost: lock the vqs one by one") moved the vq
>> lock to improve scalability, but introduced a possible deadlock in
>> vhost-iotlb. vhost_iotlb_notify_vq() now takes vq->mutex while holding
>> the device's IOTLB spinlock.
>
> Indeed spin_lock is just outside this snippet. Yack.
>
>> And on the vhost_iotlb_miss() path, the
>> spinlock is taken while holding vq->mutex.
>>
>> As long as we hold dev->mutex to prevent an ioctl from modifying
>> vq->poll concurrently, we can safely call vhost_poll_queue() without
>> holding vq->mutex. Since vhost_process_iotlb_msg() holds dev->mutex when
>> calling vhost_iotlb_notify_vq(), avoid the deadlock by not taking
>> vq->mutex.
>>
>> Fixes: 78139c94dc8c ("net: vhost: lock the vqs one by one")
>> Signed-off-by: Jean-Philippe Brucker <jean-philippe.bruc...@arm.com>
>
>
> Acked-by: Michael S. Tsirkin <m...@redhat.com>
>
> but see below for a minor comment.
>
> I guess we now need this on stable?
I don't think so, the bug is introduced in 4.20
>
>> ---
>> drivers/vhost/vhost.c | 6 +++---
>> 1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 3a5f81a66d34..1cbb17f898f7 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -944,10 +944,10 @@ static void vhost_iotlb_notify_vq(struct vhost_dev *d,
>> if (msg->iova <= vq_msg->iova &&
>> msg->iova + msg->size - 1 >= vq_msg->iova &&
>> vq_msg->type == VHOST_IOTLB_MISS) {
>> - mutex_lock(&node->vq->mutex);
>> + /* Safe to call outside vq->mutex as long as dev->mutex
>> + * is held.
>> + */
>> vhost_poll_queue(&node->vq->poll);
>> - mutex_unlock(&node->vq->mutex);
>> -
>
> In fact vhost_poll_queue is generally lockless so it's
> safe to call without any locks.
Right, I'll remove the comment
Thanks,
Jean
>
>
>> list_del(&node->node);
>> kfree(node);
>
>> }
>> --
>> 2.19.1
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