Re: [f2fs-dev] [PATCH 1/2] f2fs: avoid deadlock caused by lock order of page and lock_op

[Chao Yu <chao@kernel.org>]

On 2017/7/1 15:28, Jaegeuk Kim wrote:
> On 06/26, Chao Yu wrote:
>> Hi Jaegeuk,
>>
>> On 2017/6/26 22:54, Jaegeuk Kim wrote:
>>> Hi Chao,
>>>
>>> On 06/26, Chao Yu wrote:
>>>> Hi Jaegeuk,
>>>>
>>>> On 2017/6/25 0:25, Jaegeuk Kim wrote:
>>>>> - punch_hole
>>>>>  - fill_zero
>>>>>   - f2fs_lock_op
>>>>>   - get_new_data_page
>>>>>    - lock_page
>>>>>
>>>>> - f2fs_write_data_pages
>>>>>  - lock_page
>>>>>  - do_write_data_page
>>>>>   - f2fs_lock_op
>>>>
>>>> Good catch!
>>>>
>>>> With this implementation, page writeback can fail due to concurrent checkpoint,
>>>> this will make fsync/atomic_commit which trigger synchronous write failed randomly.
>>>>
>>>> How about unifying the lock order in punch_hole as one in writepages for regular
>>>> inode? We can add one more parameter in get_new_data_page to indicate whether
>>>> callee needs to lock cp_rwsem.
>>>
>>> Currently, there would be some places to keep cp_rwsem -> page.lock, which seems
>>> not simple to change the lock order with page.lock -> cp_rwsem. IMO, we can retry
>>> flushing data in f2fs_sync_file, once it gets -EAGAIN.
>>>
>>> Any thoughts?
>>
>> What about adding inode_lock in f2fs_sync_file to exclude other
>> foreground operation which have reversed lock order? Atomic_commit is OK
>> since it has inode_lock in its path.
> 
> I have concerned about performance regression, if we do that.

I think fsync vs write or fsync vs fsync scenarios are unusual, so is
there any usecase?

Thanks,

> 
>>
>> Thanks,
>>
>>>
>>>>
>>>> Thanks,
>>>>
>>>>>
>>>>> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
>>>>> ---
>>>>>  fs/f2fs/data.c | 5 +++--
>>>>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>>>>> index 7d3af48d34a9..9141bd19a902 100644
>>>>> --- a/fs/f2fs/data.c
>>>>> +++ b/fs/f2fs/data.c
>>>>> @@ -1404,8 +1404,9 @@ int do_write_data_page(struct f2fs_io_info *fio)
>>>>>  		}
>>>>>  	}
>>>>>  
>>>>> -	if (fio->need_lock == LOCK_REQ)
>>>>> -		f2fs_lock_op(fio->sbi);
>>>>> +	/* Deadlock due to between page->lock and f2fs_lock_op */
>>>>> +	if (fio->need_lock == LOCK_REQ && !f2fs_trylock_op(fio->sbi))
>>>>> +		return -EAGAIN;
>>>>>  
>>>>>  	err = get_dnode_of_data(&dn, page->index, LOOKUP_NODE);
>>>>>  	if (err)
>>>>>
>>>
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