Re: [PATCH v8 2/4] i2c-smbus: add SMBus Host Notify support

[Benjamin Tissoires]

On Jul 18 2016 or thereabouts, Jean Delvare wrote:
> Hi Benjamin, Wolfram,
> 
> Now that I have reviewed the i2c-i801 part of the implementation, I'm
> wondering...
> 
> On Thu,  9 Jun 2016 16:53:48 +0200, Benjamin Tissoires wrote:
> > +/**
> > + * i2c_setup_smbus_host_notify - Allocate a new smbus_host_notify for the given
> > + * I2C adapter.
> > + * @adapter: the adapter we want to associate a Host Notify function
> > + *
> > + * 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.
> > + */
> > +struct smbus_host_notify *i2c_setup_smbus_host_notify(struct i2c_adapter *adap)
> > +{
> > +	struct smbus_host_notify *host_notify;
> > +
> > +	host_notify = devm_kzalloc(&adap->dev, sizeof(struct smbus_host_notify),
> > +				   GFP_KERNEL);
> > +	if (!host_notify)
> > +		return NULL;
> > +
> > +	host_notify->adapter = adap;
> > +
> > +	spin_lock_init(&host_notify->lock);
> > +	INIT_WORK(&host_notify->work, smbus_host_notify_work);
> 
> Here we initialize a workqueue.
> 
> > +
> > +	return host_notify;
> > +}
> > +EXPORT_SYMBOL_GPL(i2c_setup_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
> > + * @data: the Host Notify data which contains the payload and address of the
> > + * client
> > + * Context: can't sleep
> > + *
> > + * Helper function to be called from an I2C bus driver's interrupt
> > + * handler. It will schedule the Host Notify work, in turn calling the
> > + * corresponding I2C device driver's alert function.
> > + *
> > + * host_notify should be a valid pointer previously returned by
> > + * i2c_setup_smbus_host_notify().
> > + */
> > +int i2c_handle_smbus_host_notify(struct smbus_host_notify *host_notify,
> > +				 unsigned short addr, unsigned int data)
> > +{
> > +	unsigned long flags;
> > +	struct i2c_adapter *adapter;
> > +
> > +	if (!host_notify || !host_notify->adapter)
> > +		return -EINVAL;
> > +
> > +	adapter = host_notify->adapter;
> > +
> > +	spin_lock_irqsave(&host_notify->lock, flags);
> > +
> > +	if (host_notify->pending) {
> > +		spin_unlock_irqrestore(&host_notify->lock, flags);
> > +		dev_warn(&adapter->dev, "Host Notify already scheduled.\n");
> > +		return -EBUSY;
> > +	}
> > +
> > +	host_notify->payload = data;
> > +	host_notify->addr = addr;
> > +
> > +	/* Mark that there is a pending notification and release the lock */
> > +	host_notify->pending = true;
> > +	spin_unlock_irqrestore(&host_notify->lock, flags);
> > +
> > +	return schedule_work(&host_notify->work);
> 
> And here we use it.
> 
> > +}
> > +EXPORT_SYMBOL_GPL(i2c_handle_smbus_host_notify);
> 
> But what happens on i2c_adapter removal? What prevents the following
> sequence from happening?
> 
> 1* A Host Notify event happens.
> 2* The event is handled and queued by i2c_handle_smbus_host_notify().
> 3* Someone tears down the underlying i2c_adapter (for example "rmmod
>    i2c-i801".)
> 4* The workqueue is processed, accessing memory which has already been
>    freed.
> 
> Of course it would be back luck, but that's pretty much the definition
> of a race condition ;-)

Yes, you are right :(
Sorry for not doing things properly :/

> 
> To be on the safe side, don't we need a teardown function in i2c-smbus,
> that could be called before i2c_del_adapter, which would remove the
> host notify handle and flush the workqueue?

I was thinking at adding a devm action on the release of the struct
smbus_host_notify, but it's actually a bad idea because some other
resources (children moslty) might already be released when the devres
action will be called.

I think it might be easier to add a i2c_remove_host_notify() (or such)
which would make sure we call the cancel_work_sync() function. It would
be the responsibility of the caller to call it once
i2c_setup_smbus_host_notify() has been called. I'd say it has the
advantage of not adding any hidden data in the adapter to the cost of a
small pain in the adapter driver.

Cheers,
Benjamin

> 
> -- 
> Jean Delvare
> SUSE L3 Support