linux-kernel - Re: [PATCH] ksmbd: use F_SETLK to force vfs_file

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Date:   Sat, 25 Dec 2021 08:08:46 +0900
From:   Namjae Jeon <linkinjeon@...nel.org>
To:     Vasily Averin <vvs@...tuozzo.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steve French <sfrench@...ba.org>,
        Hyunchul Lee <hyc.lee@...il.com>, kernel@...nvz.org,
        linux-cifs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ksmbd: use F_SETLK to force vfs_file_lock() to return asynchronously

2021-12-24 21:31 GMT+09:00, Vasily Averin <vvs@...tuozzo.com>:
> On 22.12.2021 18:17, Vasily Averin wrote:
>> On 22.12.2021 11:58, Namjae Jeon wrote:
>>> 2021-12-22 15:51 GMT+09:00, Vasily Averin <vvs@...tuozzo.com>:
>>>> On 22.12.2021 08:25, Namjae Jeon wrote:
>>>>> 2021-12-22 13:32 GMT+09:00, Vasily Averin <vvs@...tuozzo.com>:
>>>>>> On 22.12.2021 05:50, Namjae Jeon wrote:
>>>>>>> 2021-12-21 22:08 GMT+09:00, Vasily Averin <vvs@...tuozzo.com>:
>>>>>>>> On 21.12.2021 15:02, Namjae Jeon wrote:
>>>>>>>>> 2021-12-19 18:34 GMT+09:00, Vasily Averin <vvs@...tuozzo.com>:
>>>>>>>>>> To avoid possible deadlock ksmbd should process locks
>>>>>>>>>> asynchronously.
>>>>>>>>>> Callers expecting vfs_file_locks() to return asynchronously
>>>>>>>>>> should
>>>>>>>>>> only
>>>>>>>>>> use F_SETLK, not F_SETLKW.
>>>>>>>>> Should I check this patch instead of
>>>>>>>>> [PATCH] ksmbd: force "fail immediately" flag on fs with its own
>>>>>>>>> ->lock
>>>>>>>>> ?
>>>>>>>>
>>>>>>>> no, these patches are independent and both ones are required.
>>>>>>>> current patch fixes incorrect kernel thread behaviour:
>>>>>>>> kernel threads should not use F_SETLKW for locking requests.
>>>>>>> How does this patch work? posix_lock_file in vfs_lock_file() does
>>>>>>> not
>>>>>>> use
>>>>>>> cmd.
>>>>>>> And your patch still leaves FL_SLEEP.
>>>>>>
>>>>>> "use F_SETLK, not F_SETLKW" was copy-pasted from requirement
>>>>>> described
>>>>>> in
>>>>>> comment above vfs_lock_file().
>>>>>>
>>>>>> posix_lock_file() is not used in all ->lock() functions, and use
>>>>>> F_SETLKW
>>>>>> forces some of affected filesystem use blocking locks:
>>>>> What I'm saying is that when we apply "ksmbd: force "fail immediately"
>>>>> flag on fs with its own ->lock ", this patch is meaningless. How is
>>>>> ->lock() with F_SETLKW called?
>>>>
>>>> I got your point finally,
>>>> yes, you are right, now this cannot happen.
>>>> However I'm going to fix all affected filesystems and then revert
>>>> "ksmbd: force "fail immediately" flag on fs with its own ->lock"
>>>> When this happen and ksmbd will still use IS_SETLKW it will trigger the
>>>> problems described below.
>>> If so, You can include one patch(this patch + revert patch) in patch
>>> series for fixing ->lock of all filesystem.
>
> I've checked how smb2_lock() handles FILE_LOCK_DEFERRED returned by
> vfs_lock_file() call.
> It seems for me, working thread will be blocked in
> ksmbd_vfs_posix_lock_wait(),
> so whole ksmbd server still can deadlock. Am I wrong probably?
No, Each commands are handled by ksmbd-io kworkers.

Thanks!
>
> Thank you,
> 	Vasily Averin
>