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

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Message-ID: <YW03PSmpMkMVnHdp@shinobu>
Date:   Mon, 18 Oct 2021 17:58:37 +0900
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     David Lechner <david@...hnology.com>, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] counter: drop chrdev_lock

On Mon, Oct 18, 2021 at 08:08:21AM +0200, Greg KH wrote:
> On Sun, Oct 17, 2021 at 01:55:21PM -0500, David Lechner wrote:
> > This removes the chrdev_lock from the counter subsystem. This was
> > intended to prevent opening the chrdev more than once. However, this
> > doesn't work in practice since userspace can duplicate file descriptors
> > and pass file descriptors to other processes. Since this protection
> > can't be relied on, it is best to just remove it.
> 
> Much better, thanks!
> 
> One remaining question:
> 
> > --- a/include/linux/counter.h
> > +++ b/include/linux/counter.h
> > @@ -297,7 +297,6 @@ struct counter_ops {
> >   * @events:		queue of detected Counter events
> >   * @events_wait:	wait queue to allow blocking reads of Counter events
> >   * @events_lock:	lock to protect Counter events queue read operations
> > - * @chrdev_lock:	lock to limit chrdev to a single open at a time
> >   * @ops_exist_lock:	lock to prevent use during removal
> 
> Why do you still need 2 locks for the same structure?
> 
> thanks,
> 
> greg k-h

Originally there was only the events_lock mutex. Initially I tried using
it to also limit the chrdev to a single open, but then came across a
"lock held when returning to user space" warning:
https://lore.kernel.org/linux-arm-kernel/YOq19zTsOzKA8v7c@shinobu/T/#m6072133d418d598a5f368bb942c945e46cfab9a5

Instead of losing the benefits of a mutex lock for protecting the
events, I ultimately implemented the chrdev_lock separately as an
atomic_t. If the chrdev_lock is removed, then we'll use events_lock
solely from now on for this structure.

William Breathitt Gray

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