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

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Date:   Mon, 18 Oct 2021 11:13:16 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
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 05:58:37PM +0900, William Breathitt Gray wrote:
> 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.

chrdev_lock should be removed, it doesn't really do what you think it
does, as per the thread yesterday :)

So does this mean you can also drop the ops_exist_lock?

thanks,

greg k-h