linux-kernel - Re: [PATCH 3/4] cxl/mem: Do not rely on device_add() side effects for dev_set

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Message-ID: <20210325171445.GE2356281@nvidia.com>
Date:   Thu, 25 Mar 2021 14:14:45 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-cxl@...r.kernel.org, ira.weiny@...el.com,
        vishal.l.verma@...el.com, alison.schofield@...el.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] cxl/mem: Do not rely on device_add() side effects
 for dev_set_name() failures

On Wed, Mar 24, 2021 at 02:02:01PM -0700, Dan Williams wrote:
> While device_add() will happen to catch dev_set_name() failures it is a
> broken pattern to follow given that the core may try to fall back to a
> different name.
> 
> Add explicit checking for dev_set_name() failures to be cleaned up by
> put_device(). Skip cdev_device_add() and proceed directly to
> put_device() if the name set failure.
> 
> Fixes: b39cb1052a5c ("cxl/mem: Register CXL memX devices")
> Reported-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
>  drivers/cxl/mem.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
> index e53d573ae4ab..d615f183520c 100644
> +++ b/drivers/cxl/mem.c
> @@ -1204,12 +1204,14 @@ static int cxl_mem_add_memdev(struct cxl_mem *cxlm)
>  	dev->bus = &cxl_bus_type;
>  	dev->devt = MKDEV(cxl_mem_major, cxlmd->id);
>  	dev->type = &cxl_memdev_type;
> -	dev_set_name(dev, "mem%d", cxlmd->id);
>  
>  	cdev = &cxlmd->cdev;
>  	cdev_init(cdev, &cxl_memdev_fops);
>  
> -	rc = cdev_device_add(cdev, dev);
> +	rc = dev_set_name(dev, "mem%d", cxlmd->id);
> +	if (rc == 0)
> +		rc = cdev_device_add(cdev, dev);

Success oriented flow please

This is much nicer if you split the allocation, then this flow is
clean and simple. cxl_alloc_memdev() is undone by cxl_memdev_release()

and I would reorder the code so they are above/below each other, it is
easy to check and understand when logically paired functions are on
the same screen.

static struct cxl_memdev *cxl_alloc_memdev(struct cxl_mem *cxlm)
{
	struct cxl_memdev *cxlmd;
	struct device *dev;

	cxlmd = kzalloc(sizeof(*cxlmd), GFP_KERNEL);
	if (!cxlmd)
		return PTR_ERR(-ENOMEM);

	init_completion(&cxlmd->ops_dead);
	cxlmd->cxlm = cxlm;

	rc = ida_alloc_range(&cxl_memdev_ida, 0, CXL_MEM_MAX_DEVS, GFP_KERNEL);
	if (rc < 0)
		goto err_free;
	cxlmd->id = rc;

	/*
	 * @cxlm is deallocated when the driver unbinds so operations
	 * that are using it need to hold a live reference.
	 */
	rc = percpu_ref_init(&cxlmd->ops_active, cxlmdev_ops_active_release, 0,
			     GFP_KERNEL);
	if (rc)
		goto err_id;

	dev = &cxlmd->dev;
	dev->parent = &pdev->dev;
	dev->bus = &cxl_bus_type;
	dev->devt = MKDEV(cxl_mem_major, cxlmd->id);
	dev->type = &cxl_memdev_type;
	device_initialize(dev);
	return cxlmd;

err_id:
	ida_free(&cxl_memdev_ida, cxlmd->id);
err_free:
	kfree(cxlmd);
	return PTR_ERR(rc);
}

static int cxl_mem_add_memdev(struct cxl_mem *cxlm)
{
	struct pci_dev *pdev = cxlm->pdev;
	struct cdev *cdev;
	int rc;

	cxlmd = cxl_alloc_memdev(cxlm);
	if (IS_ERR(cxlmd))
		return ERR_PTR(cxlmd)

	rc = dev_set_name(dev, "mem%d", cxlmd->id);
	if (rc)
		goto err;

	cdev = &cxlmd->cdev;
	cdev_init(cdev, &cxl_memdev_fops);

	// Must be last
	rc = cdev_device_add(cdev, dev);
	if (rc)
		goto err;

	return devm_add_action_or_reset(dev->parent, cxlmdev_unregister, cxlmd);
err:
	put_device(&cxlmd->dev);
	return rc;
}