linux-kernel - Re: [PATCH] scsi: sd: unregister device if device_add_disk() failed in sd

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Date: Thu, 21 Dec 2023 22:49:09 -0800
From: Luis Chamberlain <mcgrof@...nel.org>
To: linan666@...weicloud.com
Cc: jejb@...ux.ibm.com, martin.petersen@...cle.com,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
	linan122@...wei.com, yukuai3@...wei.com, yi.zhang@...wei.com,
	houtao1@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH] scsi: sd: unregister device if device_add_disk() failed
 in sd_probe()

On Fri, Dec 08, 2023 at 04:23:35PM +0800, linan666@...weicloud.com wrote:
> From: Li Nan <linan122@...wei.com>
> 
> "if device_add() succeeds, you should call device_del() when you want to
> get rid of it."
> 
> In sd_probe(), device_add_disk() fails when device_add() has already
> succeeded, so change put_device() to device_unregister() to ensure device
> resources are released.
> 
> Fixes: 2a7a891f4c40 ("scsi: sd: Add error handling support for add_disk()")
> Signed-off-by: Li Nan <linan122@...wei.com>

Nacked-by: Luis Chamberlain <mcgrof@...nel.org>

> ---
>  drivers/scsi/sd.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> index 542a4bbb21bc..d81cbeee06eb 100644
> --- a/drivers/scsi/sd.c
> +++ b/drivers/scsi/sd.c
> @@ -3736,7 +3736,7 @@ static int sd_probe(struct device *dev)
>  
>  	error = device_add_disk(dev, gd, NULL);
>  	if (error) {
> -		put_device(&sdkp->disk_dev);
> +		device_unregister(&sdkp->disk_dev);
>  		put_disk(gd);
>  		goto out;
>  	}

This is incorrect, device_unregister() calls:

void device_unregister(struct device *dev)                                      
{                                                                               
	pr_debug("device: '%s': %s\n", dev_name(dev), __func__);                
	device_del(dev);                                                        
	put_device(dev);                                                        
}   

So you're adding what you believe to be a correct missing device_del().
But what you missed is that if device_add_disk() fails then device_add()
did not succeed because the new code we have in the kernel *today* unwinds
this for us now.

What you missed is that in today's code inside device_add_disk(), if
device_add() succeeeds we now unwind and call device_del() for the
device for you. And so, quoting the next sentence you took from
device_add():

"If device_add() has *not* succeeded, use *only* put_device() to drop the
 reference count."

Please do reference in the future a crash dump / or explain how you
reached your conclusions if you do not have a crash dump to prove an
issue. Specially if you are suggesting it Fixes a commit.

  Luis