Re: [PATCH] counter: drop chrdev_lock

[William Breathitt Gray <vilhelm.gray@gmail.com>]

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On Mon, Oct 18, 2021 at 11:03:49AM -0500, David Lechner wrote:
> On 10/18/21 4:14 AM, William Breathitt Gray wrote:
> > On Sun, Oct 17, 2021 at 01:55:21PM -0500, David Lechner wrote:
> >> diff --git a/drivers/counter/counter-sysfs.c b/drivers/counter/counter-sysfs.c
> >> index 1ccd771da25f..7bf8882ff54d 100644
> >> --- a/drivers/counter/counter-sysfs.c
> >> +++ b/drivers/counter/counter-sysfs.c
> >> @@ -796,25 +796,18 @@ static int counter_events_queue_size_write(struct counter_device *counter,
> >>   					   u64 val)
> >>   {
> >>   	DECLARE_KFIFO_PTR(events, struct counter_event);
> >> -	int err = 0;
> >> -
> >> -	/* Ensure chrdev is not opened more than 1 at a time */
> >> -	if (!atomic_add_unless(&counter->chrdev_lock, 1, 1))
> >> -		return -EBUSY;
> >> +	int err;
> >>   
> >>   	/* Allocate new events queue */
> >>   	err = kfifo_alloc(&events, val, GFP_KERNEL);
> >>   	if (err)
> >> -		goto exit_early;
> >> +		return err;
> >>   
> >>   	/* Swap in new events queue */
> >>   	kfifo_free(&counter->events);
> >>   	counter->events.kfifo = events.kfifo;
> > 
> > Do we need to hold the events_lock mutex here for this swap in case
> > counter_chrdev_read() is in the middle of reading the kfifo to
> > userspace, or do the kfifo macros already protect us from a race
> > condition here?
> > 
> Another possibility might be to disallow changing the size while
> events are enabled. Otherwise, we also need to protect against
> write after free.
> 
> I considered this:
> 
> 	swap(counter->events.kfifo, events.kfifo);
> 	kfifo_free(&events);
> 
> But I'm not sure that would be safe enough.

I think it depends on whether it's safe to call kfifo_free() while other
kfifo_*() calls are executing. I suspect it is not safe because I don't
think kfifo_free() waits until all kfifo read/write operations are
finished before freeing -- but if I'm wrong here please let me know.

Because of that, will need to hold the counter->events_lock afterall so
that we don't modify the events fifo while a kfifo read/write is going
on, lest we suffer an address fault. This can happen regardless of
whether you swap before or after the kfifo_free() because the old fifo
address could still be in use within those uncompleted kfifo_*() calls
if they were called before the swap but don't complete before the
kfifo_free().

So we have a problem now that I think you have already noticed: the
kfifo_in() call in counter_push_events() also needs protection, but it's
executing within an interrupt context so we can't try to lock a mutex
lest we end up sleeping.

One option we have is as you suggested: we disallow changing size while
events are enabled. However, that will require us to keep track of when
events are disabled and implement a spinlock to ensure that we don't
disable events in the middle of a kfifo_in().

Alternatively, we could change events_lock to a spinlock and use it to
protect all these operations on the counter->events fifo. Would this
alternative be a better option so that we avoid creating another
separate lock?

As a side note, you can also remove the atomic.h includes from these
files now as well because we won't be working with an atomic_t anymore.

William Breathitt Gray

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