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

[Jaegeuk Kim]

On 07/05, Chao Yu wrote:
> On 2017/7/1 22:27, Jaegeuk Kim wrote:
> > 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?
> 
> Do you have test numbers of dbench/tiotest with inode:lock in fsync?

No, do we need?

> 
> Thanks,
> 
> > 
> >>
> >> Thanks,
> >>
> >>>
> >>>>
> >>>> Thanks,
> >>>>
> >>>>>
> >>>>>>
> >>>>>> Thanks,
> >>>>>>
> >>>>>>>
> >>>>>>> Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> >>>>>>> ---
> >>>>>>>  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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> >>>>> _______________________________________________
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> >>>>>