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

John Fastabend
2017-01-13T23:56:42Z

On 17-01-13 09:31 AM, John Fastabend wrote:
> On 17-01-13 08:34 AM, Stephen Hemminger wrote:
>> On Thu, 12 Jan 2017 18:51:00 -0800
>> John Fastabend <john.fastabend@gmail.com> wrote:
>>
>>>
>>> -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.
>
>>
>> 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);