On Thu, 2013-12-26 at 13:28 -0800, Michael Dalton wrote:
> On Mon, Dec 23, 2013 at 11:37 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> > So there isn't a conflict with respect to locking.
> >
> > Is it problematic to use same page_frag with both GFP_ATOMIC and with
> > GFP_KERNEL? If yes why?
>
> I believe it is safe to use the same page_frag and I will send out a
> followup patchset using just the per-receive page_frags. For future
> consideration, Eric noted that disabling NAPI before GFP_KERNEL
> allocs can potentially inhibit virtio-net network processing for some
> time (e.g., during a blocking memory allocation or preemption).
Yep, using napi_disable() in the refill process looks quite inefficient
to me, it not buggy.
napi_disable() is a big hammer, while whole idea of having a process to
block on GFP_KERNEL allocations is to allow some asynchronous behavior.
I have hard time to convince myself virtio_net is safe anyway with this
work queue thing.
virtnet_open() seems racy for example :
for (i = 0; i < vi->max_queue_pairs; i++) {
if (i < vi->curr_queue_pairs)
/* Make sure we have some buffers: if oom use wq. */
if (!try_fill_recv(&vi->rq[i], GFP_KERNEL))
schedule_delayed_work(&vi->refill, 0);
virtnet_napi_enable(&vi->rq[i]);
What if the workqueue is scheduled _before_ the call to virtnet_napi_enable(&vi->rq[i]) ?
refill_work() will happily conflict with another cpu, two cpus could
call try_fill_recv() at the same time, or worse napi_enable() would crash.
I do not have time to make a full check, but I guess there are
other races like this one.
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index c51a98867a40..b8e2adb5d0c2 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -726,16 +726,18 @@ again:
static int virtnet_open(struct net_device *dev)
{
struct virtnet_info *vi = netdev_priv(dev);
+ bool refill = false;
int i;
for (i = 0; i < vi->max_queue_pairs; i++) {
if (i < vi->curr_queue_pairs)
/* Make sure we have some buffers: if oom use wq. */
if (!try_fill_recv(&vi->rq[i], GFP_KERNEL))
- schedule_delayed_work(&vi->refill, 0);
+ refill = true;
virtnet_napi_enable(&vi->rq[i]);
}
-
+ if (refill)
+ schedule_delayed_work(&vi->refill, 0);
return 0;
}
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