Re: [PATCH] locks: add locks_move_blocks in posix_lock_inode

[yangerkun <yangerkun@huawei.com>]

在 2020/6/2 23:56, Jeff Layton 写道:
> On Tue, 2020-06-02 at 21:49 +0800, yangerkun wrote:
>>
>> 在 2020/6/2 7:10, NeilBrown 写道:
>>> On Mon, Jun 01 2020, yangerkun wrote:
>>>
>>>> We forget to call locks_move_blocks in posix_lock_inode when try to
>>>> process same owner and different types.
>>>>
>>>
>>> This patch is not necessary.
>>> The caller of posix_lock_inode() must calls locks_delete_block() on
>>> 'request', and that will remove all blocked request and retry them.
>>>
>>> So calling locks_move_blocks() here is at most an optimization.  Maybe
>>> it is a useful one.
>>>
>>> What led you to suggesting this patch?  Were you just examining the
>>> code, or was there some problem that you were trying to solve?
>>
>>
>> Actually, case of this means just replace a exists file_lock. And once
>> we forget to call locks_move_blocks, the function call of
>> posix_lock_inode will also call locks_delete_block, and will wakeup all
>> blocked requests and retry them. But we should do this until we UNLOCK
>> the file_lock! So, it's really a bug here.
>>
> 
> Waking up waiters to re-poll a lock that's still blocked seems wrong. I
> agree with Neil that this is mainly an optimization, but it does look
> useful.

Agree. Logic of this seems wrong, but it won't trigger any problem since
the waiters will conflict and try wait again.

> 
> Unfortunately this is the type of thing that's quite difficult to test
> for in a userland testcase. Is this something you noticed due to the
> extra wakeups or did you find it by inspection? It'd be great to have a
> better way to test for this in xfstests or something.

Notice this after reading the patch 5946c4319ebb ("fs/locks: allow a
lock request to block other requests."), and find that we have do the
same thing exist in flock_lock_inode and another place exists in
posix_lock_inode.

> 
> I'll plan to add this to linux-next. It should make v5.9, but let me
> know if this is causing real-world problems and maybe we can make a case
> for v5.8.

Actually, I have not try to find will this lead to some real-world
problems... Sorry for this.:(


Thanks,
Kun.

> 
> Thanks,
> Jeff
> 
>>>
>>>> Signed-off-by: yangerkun <yangerkun@huawei.com>
>>>> ---
>>>>    fs/locks.c | 1 +
>>>>    1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/fs/locks.c b/fs/locks.c
>>>> index b8a31c1c4fff..36bd2c221786 100644
>>>> --- a/fs/locks.c
>>>> +++ b/fs/locks.c
>>>> @@ -1282,6 +1282,7 @@ static int posix_lock_inode(struct inode *inode, struct file_lock *request,
>>>>    				if (!new_fl)
>>>>    					goto out;
>>>>    				locks_copy_lock(new_fl, request);
>>>> +				locks_move_blocks(new_fl, request);
>>>>    				request = new_fl;
>>>>    				new_fl = NULL;
>>>>    				locks_insert_lock_ctx(request, &fl->fl_list);
>>>> -- 
>>>> 2.21.3
>