Re: [RFC][PATCH] vhost/vsock: Add vsock_list file to map cid with vhost tasks

On Fri, 7 May 2021 17:43:32 +0200
Stefano Garzarella [off-list ref] wrote:

quoted

The start/stop of a seq_file() is made for taking locks. I do this with all
my code in ftrace. Yeah, there's a while loop between the two, but that's
just to fill the buffer. It's not that long and it never goes to userspace
between the two. You can even use this for spin locks (but I wouldn't
recommend doing it for raw ones).  

Ah okay, thanks for the clarification!

I was worried because building with `make C=2` I had these warnings:

../drivers/vhost/vsock.c:944:13: warning: context imbalance in 'vsock_start' - wrong count at exit
../drivers/vhost/vsock.c:963:13: warning: context imbalance in 'vsock_stop' - unexpected unlock

Maybe we need to annotate the functions somehow.

Yep, I it should have been.

static void *vsock_start(struct seq_file *m, loff_t *pos)
	__acquires(rcu)
{
	[...]

}

static void vsock_stop(struct seq_file *m, void *p)
	__releases(rcu)
{
	[...]
}

static int vsock_show(struct seq_file *m, void *v)
	__must_hold(rcu)
{
	[...]
}


And guess what? I just copied those annotations from sock_hash_seq_start(),
sock_hash_seq_show() and sock_hash_seq_stop() from net/core/sock_map.c
which is doing exactly the same thing ;-)

So there's definitely precedence for this.

quoted

quoted

quoted

+
+	iter->index = -1;
+	iter->node = NULL;
+	t = vsock_next(m, iter, NULL);
+
+	for (; iter->index < HASH_SIZE(vhost_vsock_hash) && l < *pos;
+	     t = vsock_next(m, iter, &l))
+		;  

A while() maybe was more readable...  

Again, I just cut and pasted from my other code.

If you have a good idea on how to implement this with netlink (something
that ss or netstat can dislpay), I think that's the best way to go.  

Okay, I'll take a look and get back to you.
If it's too complicated, we can go ahead with this patch.

Awesome, thanks!

-- Steve
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