[PATCHv6 01/10] media: add CEC notifier support

[Russell King - ARM Linux]

On Sat, Apr 01, 2017 at 11:22:03AM +0200, Hans Verkuil wrote:
> On 31/03/17 22:46, Russell King - ARM Linux wrote:
> > On Fri, Mar 31, 2017 at 02:20:27PM +0200, Hans Verkuil wrote:
> >> +struct cec_notifier *cec_notifier_get(struct device *dev)
> >> +{
> >> +	struct cec_notifier *n;
> >> +
> >> +	mutex_lock(&cec_notifiers_lock);
> >> +	list_for_each_entry(n, &cec_notifiers, head) {
> >> +		if (n->dev == dev) {
> >> +			mutex_unlock(&cec_notifiers_lock);
> >> +			kref_get(&n->kref);
> > 
> > Isn't this racy?  What stops one thread trying to get the notifier
> > while another thread puts the notifier?
> > 
> 
> Both get and put take the global cec_notifiers_lock mutex.

No, that doesn't help:

Thread 0			Thread 1
mutex_lock()
list_for_each_entry()
if()
mutex_unlock()
				mutex_lock()
				kref_put()
					list_del()
					kfree()
				mutex_unlock()
kref_get()

So, it's possible that kref_get() can be called on kfree'd memory.

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