linux-kernel - Re: [PATCH 1/3] cdev: Finish the cdev api with queued mode support

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Date:   Wed, 20 Jan 2021 12:50:43 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Dan Williams <dan.j.williams@...el.com>, gregkh@...uxfoundation.org
Cc:     Hans Verkuil <hans.verkuil@...co.com>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Dave Jiang <dave.jiang@...el.com>, vishal.l.verma@...el.com,
        linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org
Subject: Re: [PATCH 1/3] cdev: Finish the cdev api with queued mode support




On 2021-01-20 12:38 p.m., Dan Williams wrote:
> ...common reference count handling scenarios were addressed, but the
> shutdown-synchronization problem was only mentioned as something driver
> developers need to be aware in the following note:
> 
>     NOTE: This guarantees that associated sysfs callbacks are not running
>     or runnable, however any cdevs already open will remain and their fops
>     will still be callable even after this function returns.
> 
> Remove that responsibility from driver developers with the concept of a
> 'queued' mode for cdevs.

I find the queued name confusing. What's being queued?

> +static const struct file_operations cdev_queued_fops = {
> +	.owner = THIS_MODULE,
> +	.open = cdev_queued_open,
> +	.unlocked_ioctl = cdev_queued_ioctl,
> +	.compat_ioctl = compat_ptr_ioctl,
> +	.llseek = noop_llseek,
> +};

Why do we only protect these fops? I'd find it a bit confusing to have
ioctl protected from use after del, but not write/read/etc.

Logan