netdev - Re: [PATCH net-next 2/3] virtio-net: use per-receive queue page frag alloc for mergeable bufs

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Message-ID: <1388094991.12212.34.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 26 Dec 2013 13:56:31 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michael Dalton <mwdalton@...gle.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Eric Dumazet <edumazet@...gle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	lf-virt <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH net-next 2/3] virtio-net: use per-receive queue page
 frag alloc for mergeable bufs

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@...hat.com> 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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