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From: Stefano Garzarella <sgarzare@redhat.com>
To: David Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org, kvm@vger.kernel.org,
	stefanha@redhat.com, mst@redhat.com
Subject: Re: [PATCH 1/4] vsock/virtio: fix locking around 'the_virtio_vsock'
Date: Thu, 30 May 2019 12:27:40 +0200	[thread overview]
Message-ID: <20190530102740.nyg6akggvy2asikt@steredhat.homenet.telecomitalia.it> (raw)
In-Reply-To: <20190529.212852.1077585415381753122.davem@davemloft.net>

On Wed, May 29, 2019 at 09:28:52PM -0700, David Miller wrote:
> From: Stefano Garzarella <sgarzare@redhat.com>
> Date: Tue, 28 May 2019 12:56:20 +0200
> 
> > @@ -68,7 +68,13 @@ struct virtio_vsock {
> >  
> >  static struct virtio_vsock *virtio_vsock_get(void)
> >  {
> > -	return the_virtio_vsock;
> > +	struct virtio_vsock *vsock;
> > +
> > +	mutex_lock(&the_virtio_vsock_mutex);
> > +	vsock = the_virtio_vsock;
> > +	mutex_unlock(&the_virtio_vsock_mutex);
> > +
> > +	return vsock;
> 
> This doesn't do anything as far as I can tell.
> 
> No matter what, you will either get the value before it's changed or
> after it's changed.
> 
> Since you should never publish the pointer by assigning it until the
> object is fully initialized, this can never be a problem even without
> the mutex being there.
> 
> Even if you sampled the the_virtio_sock value right before it's being
> set to NULL by the remove function, that still can happen with the
> mutex held too.

Yes, I found out when I was answering Jason's question [1]. :(

I proposed to use RCU to solve this issue, do you agree?
Let me know if there is a better way.

> 
> This function is also terribly named btw, it implies that a reference
> count is being taken.  But that's not what this function does, it
> just returns the pointer value as-is.

What do you think if I remove the function, using directly the_virtio_vsock?
(should be easier to use with RCU API)

Thanks,
Stefano

[1] https://lore.kernel.org/netdev/20190529105832.oz3sagbne5teq3nt@steredhat

  reply	other threads:[~2019-05-30 10:27 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-28 10:56 [PATCH 0/4] vsock/virtio: several fixes in the .probe() and .remove() Stefano Garzarella
2019-05-28 10:56 ` [PATCH 1/4] vsock/virtio: fix locking around 'the_virtio_vsock' Stefano Garzarella
2019-05-30  4:28   ` David Miller
2019-05-30 10:27     ` Stefano Garzarella [this message]
2019-05-28 10:56 ` [PATCH 2/4] vsock/virtio: stop workers during the .remove() Stefano Garzarella
2019-05-28 10:56 ` [PATCH 3/4] vsock/virtio: fix flush of works " Stefano Garzarella
2019-05-29  3:22   ` Jason Wang
2019-05-29 10:58     ` Stefano Garzarella
2019-05-30  9:46       ` Jason Wang
2019-05-30 10:10         ` Stefano Garzarella
2019-05-30 11:59           ` Jason Wang
2019-05-31  8:18             ` Stefano Garzarella
2019-05-31  9:56               ` Jason Wang
2019-06-06  8:11                 ` Stefano Garzarella
2019-06-13  8:57                   ` Jason Wang
2019-06-27 10:26                     ` Stefano Garzarella
2019-05-28 10:56 ` [PATCH 4/4] vsock/virtio: free used buffers " Stefano Garzarella
2019-06-10 13:09 ` [PATCH 0/4] vsock/virtio: several fixes in the .probe() and .remove() Stefan Hajnoczi
2019-06-27 10:05   ` Stefano Garzarella

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