Message-ID: <20160729163025.GQ19383@mail.corp.redhat.com>
Date:	Fri, 29 Jul 2016 18:30:25 +0200
From:	Benjamin Tissoires <benjamin.tissoires@...hat.com>
To:	Jean Delvare <jdelvare@...e.de>
Cc:	Wolfram Sang <wsa@...-dreams.de>, linux-i2c@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] i2c: i2c-smbus: prevent races on remove when Host
 Notify is used

Salut Jean,

On Jul 29 2016 or thereabouts, Jean Delvare wrote:
> Salut Benjamin,
> 
> On Thu, 28 Jul 2016 11:50:39 +0200, Benjamin Tissoires wrote:
> > struct host_notify contains its own workqueue, so there is a race when
> > the adapter gets removed:
> > - the adapter schedules a notification
> > - the notification is on hold
> > - the adapter gets removed and all its children too
> > - the worker fires and access illegal memory
> > 
> > Add an API to actually kill the workqueue and prevent it to access such
> > illegal memory. I couldn't find a reliable way of automatically calling
> > this, so it's the responsibility of the adapter driver to clean up after
> > itself.
> 
> I'm no expert on the matter but I wonder if you could not just add it
> to i2c_adapter_dev_release()?

Hmm, 2 reasons I'd rather not:
1. in i2c_adapter_dev_release(), we already have removed the children
   (and called device_unregister() on the clients attached)
2. This would add a hard dependency on i2c-smbus and in the end the
   extension will get compiled in the kernel and not as a module anymore

For 1., we could also call this cleanup at the very beginning of
i2c_del_adapter(), but it feels weird to manually cancel this behind the
back of the driver.

For 2., that's something arguable.

Plus with this patche's implementation, it forces the driver (i2c-i801)
to unset the Host Notify bit which prevents new notifications to come in.

Thanks for the reviews on the other patches.

Cheers,
Benjamin

> 
> > Signed-off-by: Benjamin Tissoires <benjamin.tissoires@...hat.com>
> > ---
> >  drivers/i2c/busses/i2c-i801.c | 14 ++++++++++++++
> >  drivers/i2c/i2c-smbus.c       | 18 ++++++++++++++++++
> >  include/linux/i2c-smbus.h     |  1 +
> >  3 files changed, 33 insertions(+)
> > 
> > diff --git a/drivers/i2c/busses/i2c-i801.c b/drivers/i2c/busses/i2c-i801.c
> > index 5ef9b73..cde9a7c 100644
> > --- a/drivers/i2c/busses/i2c-i801.c
> > +++ b/drivers/i2c/busses/i2c-i801.c
> > @@ -959,6 +959,19 @@ static int i801_enable_host_notify(struct i2c_adapter *adapter)
> >  	return 0;
> >  }
> >  
> > +static void i801_disable_host_notify(struct i2c_adapter *adapter)
> > +{
> > +	struct i801_priv *priv = i2c_get_adapdata(adapter);
> 
> You pass the adapter as the parameter, but don't need it. All you need
> is priv, which the caller has too. So you could pass priv as the
> parameter directly and avoid the glue code.
> 
> > +
> > +	if (!(priv->features & FEATURE_HOST_NOTIFY))
> > +		return;
> > +
> > +	/* disable Host Notify... */
> > +	outb_p(0, SMBSLVCMD(priv));
> 
> This assumes there's only one bit in the register, which is not true.
> There are 3 bits. I did not notice the problem during my original
> review, but in i801_enable_host_notify() you are silently zero-ing the
> other 2 bits too, which isn't nice. You should only touch the bit that
> matters to you, both here and in i801_enable_host_notify().
> 
> > +	/* ...and kill the already queued notifications */
> > +	i2c_cancel_smbus_host_notify(priv->host_notify);
> 
> I thought we would rather process them than cancel them. But I suppose
> it makes no difference if the system is being shut down anyway.
> 
> > +}
> > +
> >  static const struct i2c_algorithm smbus_algorithm = {
> >  	.smbus_xfer	= i801_access,
> >  	.functionality	= i801_func,
> > @@ -1648,6 +1661,7 @@ static void i801_remove(struct pci_dev *dev)
> >  	pm_runtime_forbid(&dev->dev);
> >  	pm_runtime_get_noresume(&dev->dev);
> >  
> > +	i801_disable_host_notify(&priv->adapter);
> >  	i801_del_mux(priv);
> >  	i2c_del_adapter(&priv->adapter);
> >  	i801_acpi_remove(priv);
> > diff --git a/drivers/i2c/i2c-smbus.c b/drivers/i2c/i2c-smbus.c
> > index b0d2679..60705dd 100644
> > --- a/drivers/i2c/i2c-smbus.c
> > +++ b/drivers/i2c/i2c-smbus.c
> > @@ -279,6 +279,8 @@ static void smbus_host_notify_work(struct work_struct *work)
> >   * Returns a struct smbus_host_notify pointer on success, and NULL on failure.
> >   * The resulting smbus_host_notify must not be freed afterwards, it is a
> >   * managed resource already.
> > + * To prevent races on remove, the caller needs to stop the embedded worker
> > + * by calling i2c_cancel_smbus_host_notify().
> >   */
> >  struct smbus_host_notify *i2c_setup_smbus_host_notify(struct i2c_adapter *adap)
> >  {
> > @@ -299,6 +301,22 @@ struct smbus_host_notify *i2c_setup_smbus_host_notify(struct i2c_adapter *adap)
> >  EXPORT_SYMBOL_GPL(i2c_setup_smbus_host_notify);
> >  
> >  /**
> > + * i2c_cancel_smbus_host_notify - Terminate any active Host Notification.
> > + * @host_notify: the host_notify object to terminate
> > + *
> > + * Cancel any pending Host Notifcation. Must be called to ensure no races
> > + * between the adaptor being removed and the Host Notify process being treated.
> 
> "... and the Host Notification being processed." would sound better
> IMHO.
> 
> > + */
> > +void i2c_cancel_smbus_host_notify(struct smbus_host_notify *host_notify)
> > +{
> > +	if (!host_notify)
> > +		return;
> 
> Can this realistically happen (I mean without being a bug in the
> driver)?
> 
> > +
> > +	cancel_work_sync(&host_notify->work);
> > +}
> > +EXPORT_SYMBOL_GPL(i2c_cancel_smbus_host_notify);
> > +
> > +/**
> >   * i2c_handle_smbus_host_notify - Forward a Host Notify event to the correct
> >   * I2C client.
> >   * @host_notify: the struct host_notify attached to the relevant adapter
> > diff --git a/include/linux/i2c-smbus.h b/include/linux/i2c-smbus.h
> > index c2e3324..ac02827 100644
> > --- a/include/linux/i2c-smbus.h
> > +++ b/include/linux/i2c-smbus.h
> > @@ -76,5 +76,6 @@ struct smbus_host_notify {
> >  struct smbus_host_notify *i2c_setup_smbus_host_notify(struct i2c_adapter *adap);
> >  int i2c_handle_smbus_host_notify(struct smbus_host_notify *host_notify,
> >  				 unsigned short addr, unsigned int data);
> > +void i2c_cancel_smbus_host_notify(struct smbus_host_notify *host_notify);
> >  
> >  #endif /* _LINUX_I2C_SMBUS_H */
> 
> 
> -- 
> Jean Delvare
> SUSE L3 Support

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