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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