RE: [PATCH net v2] vsock: Fix a lockdep warning in __vsock_release()

[Dexuan Cui <decui@microsoft.com>]

> From: linux-hyperv-owner@vger.kernel.org
> <linux-hyperv-owner@vger.kernel.org> On Behalf Of Stefano Garzarella
> Sent: Thursday, September 26, 2019 12:48 AM
> 
> Hi Dexuan,
> 
> On Thu, Sep 26, 2019 at 01:11:27AM +0000, Dexuan Cui wrote:
> > ...
> > NOTE: I only tested the code on Hyper-V. I can not test the code for
> > virtio socket, as I don't have a KVM host. :-( Sorry.
> >
> > @Stefan, @Stefano: please review & test the patch for virtio socket,
> > and let me know if the patch breaks anything. Thanks!
> 
> Comment below, I'll test it ASAP!

Stefano, Thank you!

BTW, this is how I tested the patch:
1. write a socket server program in the guest. The program calls listen()
and then calls sleep(10000 seconds). Note: accept() is not called.

2. create some connections to the server program in the guest.

3. kill the server program by Ctrl+C, and "dmesg" will show the scary
call-trace, if the kernel is built with 
	CONFIG_LOCKDEP=y
	CONFIG_LOCKDEP_SUPPORT=y

4. Apply the patch, do the same test and we should no longer see the call-trace.

> > -		lock_sock(sk);
> > +		/* When "level" is 2, use the nested version to avoid the
> > +		 * warning "possible recursive locking detected".
> > +		 */
> > +		if (level == 1)
> > +			lock_sock(sk);
> 
> Since lock_sock() calls lock_sock_nested(sk, 0), could we use directly
> lock_sock_nested(sk, level) with level = 0 in vsock_release() and
> level = SINGLE_DEPTH_NESTING here in the while loop?
> 
> Thanks,
> Stefano

IMHO it's better to make the lock usage more explicit, as the patch does.

lock_sock_nested(sk, level) or lock_sock_nested(sk, 0) seems a little
odd to me. But I'm open to your suggestion: if any of the network
maintainers, e.g. davem, also agrees with you, I'll change the code 
as you suggested. :-)

Thanks,
-- Dexuan