linux-kernel - Re: [PATCH 3/3] zonefs: convert to use kobject_is

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Message-ID: <d732a8f6-4a0a-d7ff-af9c-f377fefd1283@fastmail.com>
Date:   Thu, 6 Apr 2023 19:58:38 +0900
From:   Damien Le Moal <dlemoal@...tmail.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Yangtao Li <frank.li@...o.com>, xiang@...nel.org, chao@...nel.org,
        huyue2@...lpad.com, jefflexu@...ux.alibaba.com,
        damien.lemoal@...nsource.wdc.com, naohiro.aota@....com,
        jth@...nel.org, rafael@...nel.org, linux-erofs@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 3/3] zonefs: convert to use kobject_is_added()

On 4/6/23 19:26, Greg KH wrote:
> On Thu, Apr 06, 2023 at 07:13:38PM +0900, Damien Le Moal wrote:
>> On 4/6/23 19:05, Greg KH wrote:
>>> On Thu, Apr 06, 2023 at 05:30:56PM +0800, Yangtao Li wrote:
>>>> Use kobject_is_added() instead of local `s_sysfs_registered` variables.
>>>> BTW kill kobject_del() directly, because kobject_put() actually covers
>>>> kobject removal automatically.
>>>>
>>>> Signed-off-by: Yangtao Li <frank.li@...o.com>
>>>> ---
>>>>  fs/zonefs/sysfs.c  | 11 +++++------
>>>>  fs/zonefs/zonefs.h |  1 -
>>>>  2 files changed, 5 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/fs/zonefs/sysfs.c b/fs/zonefs/sysfs.c
>>>> index 8ccb65c2b419..f0783bf7a25c 100644
>>>> --- a/fs/zonefs/sysfs.c
>>>> +++ b/fs/zonefs/sysfs.c
>>>> @@ -101,8 +101,6 @@ int zonefs_sysfs_register(struct super_block *sb)
>>>>  		return ret;
>>>>  	}
>>>>  
>>>> -	sbi->s_sysfs_registered = true;
>>>
>>> You know this, why do you need to have a variable tell you this or not?
>>
>> If kobject_init_and_add() fails, zonefs_sysfs_register() returns an error and
>> fill_super will also return that error. vfs will then call kill_super, which
>> calls zonefs_sysfs_unregister(). For that case, we need to know that we actually
>> added the kobj.
> 
> Ok, but then why not just 0 out the kobject pointer here instead?  That
> way you will always know if it's a valid pointer or not and you don't
> have to rely on some other variable?  Use the one that you have already :)

but sbi->s_kobj is the kobject itself, not a pointer. I can still zero it out in
case of error to avoid using the added s_sysfs_registered bool. I would need to
check a field of s_kobj though, which is not super clean and makes the code
dependent on kobject internals. Not super nice in my opinion, unless I am
missing something.

> And you really don't even need to check anything, just pass in NULL to
> kobject_del() and friends, it should handle it.>
>>>> -
>>>>  	return 0;
>>>>  }
>>>>  
>>>> @@ -110,12 +108,13 @@ void zonefs_sysfs_unregister(struct super_block *sb)
>>>>  {
>>>>  	struct zonefs_sb_info *sbi = ZONEFS_SB(sb);
>>>>  
>>>> -	if (!sbi || !sbi->s_sysfs_registered)
>>>
>>> How can either of these ever be true?  Note, sbi should be passed here
>>> to this function, not the super block as that is now unregistered from
>>> the system.  Looks like no one has really tested this codepath that much
>>> :(
>>>
>>>> +	if (!sbi)
>>>>  		return;
>>>
>>> this can not ever be true, right?
>>
>> Yes it can, if someone attempt to mount a non zoned device. In that case,
>> fill_super returns early without setting sb->s_fs_info but vfs still calls
>> kill_super.
> 
> But you already had a sbi pointer in the place that this was called, so
> you "know" if you need to even call into here or not.  You are having to
> look up the same pointer multiple times in this call chain, there's no
> need for that.

I am not following here. Either we check that we have sbi here in
zonefs_sysfs_unregister(), or we conditionally call this function in
zonefs_kill_super() with a "if (sbi)". Either way, we need to check since sbi
can be NULL.

> 
> thanks,
> 
> greg k-h