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From:   Dexuan Cui <decui@microsoft.com>
To:     Stefano Garzarella <sgarzare@redhat.com>
CC:     "davem@davemloft.net" <davem@davemloft.net>,
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Subject: RE: [PATCH net v2] vsock: Fix a lockdep warning in __vsock_release()
Thread-Topic: [PATCH net v2] vsock: Fix a lockdep warning in __vsock_release()
Thread-Index: AQHVdD68klVN4bkfhUuYdmaenlvLyqc+f72g
Date:   Fri, 27 Sep 2019 05:37:20 +0000
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> 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
>=20
> Hi Dexuan,
>=20
> 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!
>=20
> 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=20
	CONFIG_LOCKDEP=3Dy
	CONFIG_LOCKDEP_SUPPORT=3Dy

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

> > -		lock_sock(sk);
> > +		/* When "level" is 2, use the nested version to avoid the
> > +		 * warning "possible recursive locking detected".
> > +		 */
> > +		if (level =3D=3D 1)
> > +			lock_sock(sk);
>=20
> Since lock_sock() calls lock_sock_nested(sk, 0), could we use directly
> lock_sock_nested(sk, level) with level =3D 0 in vsock_release() and
> level =3D SINGLE_DEPTH_NESTING here in the while loop?
>=20
> 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=20
as you suggested. :-)

Thanks,
-- Dexuan