Re: [PATCH 2/2] vhost: Avoid that vhost_work_flush() returns early

Michael S. Tsirkin
2013-08-14T11:45:05Z

On Wed, Aug 14, 2013 at 09:03:28AM +0200, Bart Van Assche wrote:
> If two or more items are queued on dev->work_list before
> vhost_worker() starts processing these then the value of
> work->done_seq will be set to the sequence number of a work item
> that has not yet been processed. Avoid this by letting
> vhost_worker() count the number of items that have already been
> processed.
>
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Asias He <asias@redhat.com>

I'm confused by this explanation.
done_seq is set here:
if (work) {
work->done_seq = seq;
if (work->flushing)
wake_up_all(&work->done);
}

and work is set here:

if (!list_empty(&dev->work_list)) {
work = list_first_entry(&dev->work_list,
struct vhost_work, node);
list_del_init(&work->node);
seq = work->queue_seq;
}

this work is processed on the next line:

if (work) {
__set_current_state(TASK_RUNNING);
work->fn(work);
if (need_resched())
schedule();
}

so how do we end up with a sequence of a work item
that isn't processed?

> ---
> drivers/vhost/vhost.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index e7ffc10..11d668a 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -202,7 +202,7 @@ static int vhost_worker(void *data)
> {
> struct vhost_dev *dev = data;
> struct vhost_work *work;
> - unsigned seq;
> + unsigned seq = 0;
> mm_segment_t oldfs = get_fs();
>
> set_fs(USER_DS);
> @@ -216,14 +216,13 @@ static int vhost_worker(void *data)
> work = list_first_entry(&dev->work_list,
> struct vhost_work, node);
> list_del_init(&work->node);
> - seq = work->queue_seq;
> spin_unlock_irq(&dev->work_lock);
>
> __set_current_state(TASK_RUNNING);
> work->fn(work);
>
> spin_lock_irq(&dev->work_lock);
> - work->done_seq = seq;
> + work->done_seq = ++seq;
> if (work->flushing)
> wake_up_all(&work->done);
> }
> --
> 1.7.10.4