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

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Date: Fri, 22 Dec 2023 16:27:16 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Luis Chamberlain <mcgrof@...nel.org>, 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, yi.zhang@...wei.com, houtao1@...wei.com,
 yangerkun@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH] scsi: sd: unregister device if device_add_disk() failed
 in sd_probe()

Hi,

在 2023/12/22 14:49, Luis Chamberlain 写道:
> 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.

I'm confused here, there are two device here, one is 'sdkp->disk_dev',
one is gendisk->part0->bd_device, and the order in which they
initialize:

sd_probe
device_add(&sdkp->disk_dev) -> succeed
device_add_disk -> failed, and device_add(bd_device) did not succeed
put_device(&sdkp->disk_dev) -> device_del is missed

I don't see that if device_add_disk() fail, device_del() for
'sdkp->disk_dev'is called from anywhere. Do I missing anything?

Thanks,
Kuai

> 
> 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
> 
> .
>