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

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Date:   Wed, 22 Dec 2021 18:17:30 +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 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.

Ok. let's do it.

> But I can still not
> understand why we need to revert the patch and apply this patch.
> Maybe, I need to check your next patches.

1) it is ideologically incorrect to call F_SETLKW from kernel thread:
W here means "if a conflicting lock is held on  the  file,
then  wait  for that lock to be released". 
However this can cause server deadlock.

2) nobody handles F_SETLKW cmd. It is set only if exported file systems does not define
own ->lock() function, and so this request is processed by posix_lock_file() ignored cmd. 
So there is no sense to set it.

3) when all affected fileystem will be fixed, it will handle properly FL_SLEEP && F_SETLK combination.
this will make unnecessary the force of SMB2_LOCKFLAG_FAIL_IMMEDIATELY and drop FL_SLEEP in ksmbd.

Thank you,
	Vasily Averin