linux-kernel - Re: [PATCH] zonefs: remove unnecessary kobject

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Message-ID: <6899de08-252e-3558-fca7-b7f91b33ec61@kernel.org>
Date:   Wed, 12 Apr 2023 17:18:37 +0900
From:   Damien Le Moal <dlemoal@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Yangtao Li <frank.li@...o.com>,
        Naohiro Aota <naohiro.aota@....com>,
        Johannes Thumshirn <jth@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] zonefs: remove unnecessary kobject_del()

On 4/12/23 17:11, Greg KH wrote:
> On Wed, Apr 12, 2023 at 05:04:16PM +0900, Damien Le Moal wrote:
>> On 4/12/23 12:19, Yangtao Li wrote:
>>> kobject_put() actually covers kobject removal automatically, which is
>>> single stage removal. So kill kobject_del() directly.
>>>
>>> Signed-off-by: Yangtao Li <frank.li@...o.com>
>>> ---
>>>  fs/zonefs/sysfs.c | 1 -
>>>  1 file changed, 1 deletion(-)
>>>
>>> diff --git a/fs/zonefs/sysfs.c b/fs/zonefs/sysfs.c
>>> index 8ccb65c2b419..a535bdea1097 100644
>>> --- a/fs/zonefs/sysfs.c
>>> +++ b/fs/zonefs/sysfs.c
>>> @@ -113,7 +113,6 @@ void zonefs_sysfs_unregister(struct super_block *sb)
>>>  	if (!sbi || !sbi->s_sysfs_registered)
>>>  		return;
>>>  
>>> -	kobject_del(&sbi->s_kobj);
>>>  	kobject_put(&sbi->s_kobj);
>>>  	wait_for_completion(&sbi->s_kobj_unregister);
>>>  }
>>
>> What I am not sure about here is that if CONFIG_DEBUG_KOBJECT_RELEASE is
>> enabled, the kobj release is delayed, so the kobject will stay in sysfs
>> potentially after the umount() returns. Not exactly nice as that potentially
>> create races in user space... Not 100% sure though.
>>
>> Greg ? Any thoughts on this ?
> 
> Yes, it's all a mess :(
> 
> See the other messatges in this thread:
> 	https://lore.kernel.org/r/20230406120716.80980-1-frank.li@vivo.com
> 
> Please don't take this patch for now, this all needs to be revisited.
> 
> We have two reference counted objects with different lifespans trying to
> be embedded in the same structure, causing a mess.
> 
> But, if we split them apart, that too has issues.  I've been thinking
> about how to resolve this, but don't have any solid ideas yet, and been
> swamped with other things...
> 
> For now, let's just leave this all alone, it's not unique to this one
> filesystem, they all have the same pattern, and we need to solve them
> all properly at the same time by moving the common code into the driver
> core so that filesystems don't have to worry about this mess.

That was my thinking. Will try to think about a solution as well.
Likely, some helpers for FSes sysfs attributes somewhere in fs/*.c are needed to
not use the kobj directly as part of the fs_info structs.