linux-kernel - Re: [PATCH] counter: drop chrdev

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Message-ID: <YW5xUtWdvW5zHFx5@shinobu>
Date:   Tue, 19 Oct 2021 16:18:42 +0900
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     David Lechner <david@...hnology.com>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] counter: drop chrdev_lock

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.

Thanks,

William Breathitt Gray

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