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Date:   Tue, 19 Oct 2021 09:29:17 +0200
From:   Greg KH <gregkh@linuxfoundation.org>
To:     William Breathitt Gray <vilhelm.gray@gmail.com>
Cc:     David Lechner <david@lechnology.com>, linux-iio@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] counter: drop chrdev_lock
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On Tue, Oct 19, 2021 at 04:18:42PM +0900, William Breathitt Gray wrote:
> On Tue, Oct 19, 2021 at 09:07:48AM +0200, Greg KH wrote:
> > On Tue, Oct 19, 2021 at 03:53:08PM +0900, William Breathitt Gray wrote:
> > > 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?
> > 
> > I would recommend just having a single lock here if at all possible,
> > until you determine that there a performance problem that can be
> > measured that would require it to be split up.
> > 
> > thanks,
> > 
> > greg k-h
> 
> All right let's go with a single events_lock spinlock then. David, if
> you make those changes and submit a v2, I'll be okay with this patch and
> can provide my ack for it.

Wait, no, you need one patch to remove the atomic lock for the open
"protection" and then another one for the other locks.  The original
patch here was fine, but can be part of a patch series, don't lump them
all together into one huge change.

thanks,

greg k-h