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

[Jaegeuk Kim]

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.

> 
> 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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