linux-kernel - Re: [PATCH net-next v2] misc: Add Renesas Synchronization Management Unit (SMU) support

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Date:   Fri, 12 Feb 2021 08:39:24 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     min.li.xe@...esas.com
Cc:     derek.kiernan@...inx.com, dragan.cvetic@...inx.com, arnd@...db.de,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2] misc: Add Renesas Synchronization Management
 Unit (SMU) support

On Thu, Feb 11, 2021 at 08:16:15PM -0500, min.li.xe@...esas.com wrote:
> +static int
> +rsmu_open(struct inode *iptr, struct file *fptr)
> +{
> +	struct rsmu_cdev *rsmu;
> +
> +	rsmu = container_of(iptr->i_cdev, struct rsmu_cdev, rsmu_cdev);
> +	if (!rsmu)
> +		return -EAGAIN;
> +
> +	/* Only one open per device at a time */
> +	if (!atomic_dec_and_test(&rsmu->open_count)) {
> +		atomic_inc(&rsmu->open_count);
> +		return -EBUSY;

This does not do what you think it does, and does not prevent multiple
applications from talking to your device at the same time.

There is no need for this at all, as it does not work, sorry.  If
multiple apps talk to your device, it's their fault, not the kernel's
fault, that things go wrong.

And I thought that Arnd already told you to fix this?

thanks,

greg k-h