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

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Date:   Mon, 27 Dec 2021 10:08:58 +0300
From:   Vasily Averin <vvs@...tuozzo.com>
To:     Namjae Jeon <linkinjeon@...nel.org>
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

On 25.12.2021 02:08, Namjae Jeon wrote:
> 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.

In this case ksmbd can do not require async lock processing
and you can drop my previous patches.

Is there any difference where thread will be blocked: inside ->lock() function
of exported filesystem or in ksmbd_vfs_posix_lock_wait? I think it isn't.

Thank you,
	Vasily Averin