linux-kernel - Re: [PATCH 1/5] i2c: i2c-smbus: prevent races on remove when Host Notify is used

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Date:	Fri, 29 Jul 2016 11:12:12 +0200
From:	Jean Delvare <jdelvare@...e.de>
To:	Benjamin Tissoires <benjamin.tissoires@...hat.com>
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 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()?

> 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