linux-kernel - Re: [PATCH v17 2/9] counter: Add character device interface

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Date:   Sun, 17 Oct 2021 16:02:42 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     William Breathitt Gray <vilhelm.gray@...il.com>, jic23@...nel.org,
        linux-stm32@...md-mailman.stormreply.com, kernel@...gutronix.de,
        a.fatoum@...gutronix.de, kamel.bouhara@...tlin.com,
        gwendal@...omium.org, david@...hnology.com,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, syednwaris@...il.com,
        patrick.havelange@...ensium.com, fabrice.gasnier@...com,
        mcoquelin.stm32@...il.com, alexandre.torgue@...com,
        o.rempel@...gutronix.de, jarkko.nikula@...ux.intel.com,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [PATCH v17 2/9] counter: Add character device interface

On 17/10/2021 15:50:11+0200, Greg KH wrote:
> Note, review of this now that it has been submitted in a pull request to
> me, sorry I missed this previously...
> 
> On Wed, Sep 29, 2021 at 12:15:59PM +0900, William Breathitt Gray wrote:
> > +static int counter_chrdev_open(struct inode *inode, struct file *filp)
> > +{
> > +	struct counter_device *const counter = container_of(inode->i_cdev,
> > +							    typeof(*counter),
> > +							    chrdev);
> > +
> > +	/* Ensure chrdev is not opened more than 1 at a time */
> > +	if (!atomic_add_unless(&counter->chrdev_lock, 1, 1))
> > +		return -EBUSY;
> 
> I understand the feeling that you wish to stop userspace from doing
> this, but really, it does not work.  Eventhough you are doing this
> correctly (you should see all the other attempts at doing this), you are
> not preventing userspace from having multiple processes access this
> device node at the same time, so please, don't even attempt to stop this
> from happening.
> 
> So you can drop the atomic "lock" you have here, it's not needed at all.
> 

Could you elaborate a bit here because we've had a similar thing in the
RTC subsystem:

https://elixir.bootlin.com/linux/latest/source/drivers/rtc/dev.c#L28

And it would mean I can remove rtc->flags completely.


-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com