Re: [net PATCH v3 2/5] net: virtio: wrap rtnl_lock in test for calling with lock already held

On 17-01-13 09:31 AM, John Fastabend wrote:

On 17-01-13 08:34 AM, Stephen Hemminger wrote:

quoted

On Thu, 12 Jan 2017 18:51:00 -0800
John Fastabend [off-list ref] wrote:

quoted

 
-static void free_receive_bufs(struct virtnet_info *vi)
+static void free_receive_bufs(struct virtnet_info *vi, bool need_lock)
 {
 	struct bpf_prog *old_prog;
 	int i;
 
-	rtnl_lock();
+	if (need_lock)
+		rtnl_lock();
 	for (i = 0; i < vi->max_queue_pairs; i++) {
 		while (vi->rq[i].pages)
 			__free_pages(get_a_page(&vi->rq[i], GFP_KERNEL), 0);

@@ -1879,7 +1880,8 @@ static void free_receive_bufs(struct virtnet_info *vi)
 		if (old_prog)
 			bpf_prog_put(old_prog);
 	}
-	rtnl_unlock();
+	if (need_lock)
+		rtnl_unlock();
 }

Conditional locking is bad idea; sparse complains about it and is later source
of bugs. The more typical way of doing this in kernel is:

OK I'll use the normal form.

quoted

void _foo(some args)
{
	ASSERT_RTNL();

	...
}

void foo(some args)
{
	rtnl_lock();
	_foo(some args)
	rtnl_unlock();
}

Actually doing this without a rtnl_try_lock() is going to create two more
callbacks in virtio core just for virtio_net. All the other users do not
appear to have locking restrictions. How about the following it at least
helps in that there is no argument passing and if/else on the locks itself
but does use the if around rtnl_try_lock().

--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -1864,12 +1864,11 @@ static void virtnet_free_queues(struct virtnet_info *vi)
        kfree(vi->sq);
 }

-static void free_receive_bufs(struct virtnet_info *vi)
+static void _free_receive_bufs(struct virtnet_info *vi)
 {
        struct bpf_prog *old_prog;
        int i;

-       rtnl_lock();
        for (i = 0; i < vi->max_queue_pairs; i++) {
                while (vi->rq[i].pages)
                        __free_pages(get_a_page(&vi->rq[i], GFP_KERNEL), 0);

@@ -1879,6 +1878,12 @@ static void free_receive_bufs(struct virtnet_info *vi)
                if (old_prog)
                        bpf_prog_put(old_prog);
        }
+}
+
+static void free_receive_bufs(struct virtnet_info *vi)
+{
+       rtnl_lock();
+       _free_receive_bufs(vi);
        rtnl_unlock();
 }

@@ -2358,7 +2363,10 @@ static void remove_vq_common(struct virtnet_info *vi)
        /* Free unused buffers in both send and recv, if any. */
        free_unused_bufs(vi);

-       free_receive_bufs(vi);
+       if (rtnl_is_locked());
+               _free_receive_bufs(vi);
+       else
+               free_receive_bufs(vi);

        free_receive_page_frags(vi);