Subject: Re: [PATCH] vhost-net: Acquire device lock when releasing device

From: Sasha Levin <[email-masked]>
Date: Fri, 27 Jan 2012 19:13:41 -0500

I just noticed that it happened again, and that this patch didn't make
it's way in.

The patch below indeed fixes the problem for me. Please push it in.

On Sun, 2011-11-27 at 19:06 +0200, Michael S. Tsirkin wrote:
> On Sun, Nov 27, 2011 at 06:49:27PM +0200, Michael S. Tsirkin wrote:
> > On Fri, Nov 18, 2011 at 11:19:42AM +0200, Sasha Levin wrote:
> > > Device lock should be held when releasing a device, and specifically
> > > when calling vhost_dev_cleanup(). Otherwise, RCU complains about it:
> > > 
> > > [ 2025.642835] ===============================
> > > [ 2025.643838] [ INFO: suspicious RCU usage. ]
> > > [ 2025.645182] -------------------------------
> > > [ 2025.645927] drivers/vhost/vhost.c:475 suspicious rcu_dereference_protected() usage!
> > > [ 2025.647329]
> > > [ 2025.647330] other info that might help us debug this:
> > > [ 2025.647331]
> > > [ 2025.649042]
> > > [ 2025.649043] rcu_scheduler_active = 1, debug_locks = 1
> > > [ 2025.650235] no locks held by trinity/21042.
> > > [ 2025.650971]
> > > [ 2025.650972] stack backtrace:
> > > [ 2025.651789] Pid: 21042, comm: trinity Not tainted 3.2.0-rc2-sasha-00057-ga9098b3 #5
> > > [ 2025.653342] Call Trace:
> > > [ 2025.653792]  [<ffffffff810b4a6a>] lockdep_rcu_suspicious+0xaf/0xb9
> > > [ 2025.654916]  [<ffffffff818d4c2c>] vhost_dev_cleanup+0x342/0x3ac
> > > [ 2025.655985]  [<ffffffff818d4f18>] vhost_net_release+0x48/0x7f
> > > [ 2025.657247]  [<ffffffff811416e3>] fput+0x11e/0x1dc
> > > [ 2025.658091]  [<ffffffff8113f1ec>] filp_close+0x6e/0x79
> > > [ 2025.658964]  [<ffffffff81089ed7>] put_files_struct+0xcc/0x196
> > > [ 2025.659971]  [<ffffffff8108a034>] exit_files+0x46/0x4f
> > > [ 2025.660865]  [<ffffffff8108a716>] do_exit+0x264/0x75c
> > > [ 2025.662201]  [<ffffffff8113f490>] ? fsnotify_modify+0x60/0x68
> > > [ 2025.663260]  [<ffffffff81bbdbca>] ? sysret_check+0x2e/0x69
> > > [ 2025.664269]  [<ffffffff8108acc1>] do_group_exit+0x83/0xb1
> > > [ 2025.665448]  [<ffffffff8108ad01>] sys_exit_group+0x12/0x16
> > > [ 2025.666396]  [<ffffffff81bbdb92>] system_call_fastpath+0x16/0x1b
> > > 
> > > Cc: "Michael S. Tsirkin" <[email-masked]>
> > > Cc: [email-masked]
> > > Cc: [email-masked]
> > > Cc: [email-masked]
> > > Signed-off-by: Sasha Levin <[email-masked]>
> > > ---
> > >  drivers/vhost/net.c |    2 ++
> > >  1 files changed, 2 insertions(+), 0 deletions(-)
> > > 
> > > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > > index 882a51f..c9be601 100644
> > > --- a/drivers/vhost/net.c
> > > +++ b/drivers/vhost/net.c
> > > @@ -586,6 +586,7 @@ static int vhost_net_release(struct inode *inode, struct file *f)
> > >  	struct socket *tx_sock;
> > >  	struct socket *rx_sock;
> > >  
> > > +	mutex_lock(&n->dev.mutex);
> > >  	vhost_net_stop(n, &tx_sock, &rx_sock);
> > >  	vhost_net_flush(n);
> > >  	vhost_dev_cleanup(&n->dev);
> > > @@ -596,6 +597,7 @@ static int vhost_net_release(struct inode *inode, struct file *f)
> > >  	/* We do an extra flush before freeing memory,
> > >  	 * since jobs can re-queue themselves. */
> > >  	vhost_net_flush(n);
> > > +	mutex_unlock(&n->dev.mutex);
> > >  	kfree(n);
> > >  	return 0;
> > >  }
> > 
> > This calls fput fom release under lock which is generally a bad idea,
> > as locks become nested then. For example, consider what would happen if this
> > somehow triggers a release which in turn needs the same mutex.
> > 
> > And, we are releasing the device, so it seems better to check
> > that no one has the lock.
> > How about the following instead? What do you think?
> > 
> > -->
> > 
> > vhost: fix release path lockdep checks
> > 
> > We shouldn't hold any locks on release path. Pass a flag to
> > vhost_dev_cleanup to use the lockdep info correctly.
> > 
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > 
> > ---
> 
> Sorry, this got cut short. Here's the full patch (1st chunk was
> missing).  Does this solve the problem for you?
> 
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 8b16d16..9bba4b3 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -598,7 +598,7 @@ static int vhost_net_release(struct inode *inode, struct file *f)
>  
>  	vhost_net_stop(n, &tx_sock, &rx_sock);
>  	vhost_net_flush(n);
> -	vhost_dev_cleanup(&n->dev);
> +	vhost_dev_cleanup(&n->dev, false);
>  	if (tx_sock)
>  		fput(tx_sock->file);
>  	if (rx_sock)
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index a8c95ef..96f9769 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -424,7 +424,7 @@ long vhost_dev_reset_owner(struct vhost_dev *dev)
>  	if (!memory)
>  		return -ENOMEM;
>  
> -	vhost_dev_cleanup(dev);
> +	vhost_dev_cleanup(dev, true);
>  
>  	memory->nregions = 0;
>  	RCU_INIT_POINTER(dev->memory, memory);
> @@ -455,8 +455,8 @@ int vhost_zerocopy_signal_used(struct vhost_virtqueue *vq)
>  	return j;
>  }
>  
> -/* Caller should have device mutex */
> -void vhost_dev_cleanup(struct vhost_dev *dev)
> +/* Caller should have device mutex if and only if locked is set */
> +void vhost_dev_cleanup(struct vhost_dev *dev, bool locked)
>  {
>  	int i;
>  
> @@ -493,7 +493,8 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
>  	dev->log_file = NULL;
>  	/* No one will access memory at this point */
>  	kfree(rcu_dereference_protected(dev->memory,
> -					lockdep_is_held(&dev->mutex)));
> +					locked ==
> +						lockdep_is_held(&dev->mutex)));
>  	RCU_INIT_POINTER(dev->memory, NULL);
>  	WARN_ON(!list_empty(&dev->work_list));
>  	if (dev->worker) {
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index b3e8cc3..97e18d3 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -164,7 +164,7 @@ struct vhost_dev {
>  long vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue *vqs, int nvqs);
>  long vhost_dev_check_owner(struct vhost_dev *);
>  long vhost_dev_reset_owner(struct vhost_dev *);
> -void vhost_dev_cleanup(struct vhost_dev *);
> +void vhost_dev_cleanup(struct vhost_dev *, bool locked);
>  long vhost_dev_ioctl(struct vhost_dev *, unsigned int ioctl, unsigned long arg);
>  int vhost_vq_access_ok(struct vhost_virtqueue *vq);
>  int vhost_log_access_ok(struct vhost_dev *);

-- 

Sasha.