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

[Jaegeuk Kim <jaegeuk@kernel.org>]

On 07/01, Chao Yu wrote:
> 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?

Well, that'd be common to call multiple fsync calls at the same time.
Like dbench or tiotest?

> 
> 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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