Re: [PATCH v5 3/5] counter: Add character device interface

David Lechner
2020-10-20T15:55:04Z

>>> +static long counter_chrdev_ioctl(struct file *filp, unsigned int cmd,
>>> + unsigned long arg)
>>> +{
>>> + struct counter_device *const counter = filp->private_data;
>>> + raw_spinlock_t *const events_lock = &counter->events_lock;
>>> + unsigned long flags;
>>> + struct list_head *const events_list = &counter->events_list;
>>> + struct list_head *const next_events_list = &counter->next_events_list;
>>> +
>>> + switch (cmd) {
>>> + case COUNTER_CLEAR_WATCHES_IOCTL:
>>> + raw_spin_lock_irqsave(events_lock, flags);
>>> + counter_events_list_free(events_list);
>>> + raw_spin_unlock_irqrestore(events_lock, flags);
>>> + counter_events_list_free(next_events_list);
>>
>> I think this ioctl is doing too much. If we have to use it for both
>> stopping events and clearing the list accumulated by
>> COUNTER_SET_WATCH_IOCTL, then we have a race condition of no events
>> after clearing watches during the time we are adding new ones and
>> until we load the new ones.
>>
>> It would probably make more sense to call this ioctl
>> COUNTER_STOP_WATCHES_IOCTL and move counter_events_list_free(
>> next_events_list) to the end of COUNTER_LOAD_WATCHES_IOCTL.
>
> I don't think we will necessarily have a race condition here.
> COUNTER_CLEAR_WATCHES_IOCTL is intended to just clear the watches; e.g.
> bring us back to a clear state when some sort of job has completely
> finished and the user is no longer going to watch events for a while
> (maybe they're adjusting the conveyor for the next job or some similar
> operation).
>
> I think the scenario you're concerned about is when you need to swap
> watches in the middle of a job without losing events. In this case, you
> wouldn't need to use COUNTER_CLEAR_WATCHES_IOCTL at all. Instead, you
> would just set up the watches via COUNTER_SET_WATCH_IOCTL, and then use
> COUNTER_LOAD_WATCHES_IOCTL to perform the swap; after
> COUNTER_LOAD_WATCHES_IOCTL completes, next_events_list is empty (thanks
> to list_replace_init()) and you're ready for the next set of watches.
>

Got it. I think I missed the fact that list_replace_init() clears
next_events_list.

>>> +
>>> +static int counter_chrdev_release(struct inode *inode, struct file *filp)
>>> +{
>>> + struct counter_device *const counter = filp->private_data;
>>> + unsigned long flags;
>>> +
>>> + put_device(&counter->dev);
>>
>> put_device() should be at the end of the function in case it is the last
>> reference.
>
> put_device() shouldn't affect the counter_device events members, so I
> don't think there's a difference in this case if it's called at the
> beginning or end of the counter_chrdev_release function.
>

It isn't possible the some memory allocated with devm_kalloc() could be
be referenced after calling put_device() now or in the future?

>>> +}
>>> +EXPORT_SYMBOL_GPL(counter_push_event);
>>
>>
>>> diff --git a/drivers/counter/counter-sysfs.c b/drivers/counter/counter-sysfs.c
>>> index e66ed99dd5ea..cefef61f170d 100644
>>> --- a/drivers/counter/counter-sysfs.c
>>> +++ b/drivers/counter/counter-sysfs.c
>>
>>
>> Not sure why sysfs changes are in the chrdev patch. Are these
>> changes related somehow?
>
> Sorry, I forgot to explain this in the cover letter. The changes here
> are only useful for the character device interface. These changes
> introduce the extensionZ_name and extensionZ_width sysfs attributes.
>
> In the character device interface, extensions are selected by their id
> number, and the value returned depends on the type of data. The new
> sysfs attributes introduced here allow users to match the id of an
> extension with its name, as well as the bit width of the value returned
> so that the user knows whether to use the value_u8 or value_u64 union
> member in struct counter_event.
>

Are we sure that all value types will always be CPU-endian unsigned
integers? Or should we make an enum to describe the data type instead
of just the width?