Hi Chao, On Thu, May 07, 2020 at 02:38:39PM +0800, Chao Yu wrote: > On 2020/5/7 6:36, Gao Xiang wrote: > > On Wed, May 06, 2020 at 12:16:13PM -0700, Eric Biggers wrote: > >> On Wed, May 06, 2020 at 02:47:19PM +0800, Gao Xiang wrote: > >>> On Wed, May 06, 2020 at 09:58:22AM +0800, Gao Xiang wrote: > >>>> On Tue, May 05, 2020 at 06:24:28PM -0700, Eric Biggers wrote: > >>>>> On Wed, May 06, 2020 at 08:14:07AM +0800, Gao Xiang wrote: > >>>>>>> > >>>>>>> Actually, I think this is wrong because the fsync can be done via a > >>>>>>> file > >>>>>>> descriptor that was opened to a now-deleted link to the file. > >>>>>> > >>>>>> I'm still confused about this... > >>>>>> > >>>>>> I don't know what's wrong with this version from my limited knowledge? > >>>>>> inode itself is locked when fsyncing, so > >>>>>> > >>>>>> if the fsync inode->i_nlink == 1, this inode has only one hard link > >>>>>> (not deleted yet) and should belong to a single directory; and > >>>>>> > >>>>>> the only one parent directory would not go away (not deleted as > >>>>>> well) > >>>>>> since there are some dirents in it (not empty). > >>>>>> > >>>>>> Could kindly explain more so I would learn more about this scenario? > >>>>>> Thanks a lot! > >>>>> > >>>>> i_nlink == 1 just means that there is one non-deleted link. There can > >>>>> be links > >>>>> that have since been deleted, and file descriptors can still be open to > >>>>> them. > >>>> > >>>> Thanks for your inspiration. You are right, thanks. > >>>> > >>>> Correct my words... I didn't check f2fs code just now, it seems f2fs > >>>> doesn't > >>>> take inode_lock as some other fs like __generic_file_fsync or > >>>> ubifs_fsync. > >>>> > >>>> And i_sem locks nlink / try_to_fix_pino similarly in some extent. It > >>>> seems > >>>> no race by using d_find_alias here. Thanks again. > >>>> > >>> > >>> (think more little bit just now...) > >>> > >>> Thread 1: Thread 2 (fsync): > >>> vfs_unlink try_to_fix_pino > >>> f2fs_unlink > >>> f2fs_delete_entry > >>> f2fs_drop_nlink (i_sem, inode->i_nlink = 1) > >>> > >>> (... but this dentry still hashed) i_sem, check > >>> inode->i_nlink = 1 > >>> i_sem d_find_alias > >>> > >>> d_delete > >>> > >>> I'm not sure if fsync could still use some wrong alias by chance.. > >>> completely untested, maybe just noise... > > Another race condition could be: > > Thread 1 (fsync) Thread 2 (rename) > - f2fs_sync_fs > - try_to_fix_pino > - f2fs_rename > - down_write > - file_lost_pino > - up_write > - down_write > - file_got_pino > - up_write
Yes, IMHO, I think it could be not proper to take dir lock in fsync path anyway... I would suggest as before (if it needs to be fixed). And it seems no significant performance difference. Thanks, Gao Xiang > > Thanks, > > >>> > >> > >> Right, good observation. My patch makes it better, but it's still broken. > >> > >> I don't know how to fix it. If we see i_nlink == 1 and multiple hashed > >> dentries, there doesn't appear to be a way to distingush which one > >> corresponds > >> to the remaining link on-disk (if any; it may not even be in the dcache), > >> and > >> which correspond to links that vfs_unlink() has deleted from disk but > >> hasn't yet > >> done d_delete() on. > >> > >> One idea would be choose one, then take inode_lock_shared(dir) and do > >> __f2fs_find_entry() to check if the dentry is really still on-disk. That's > >> heavyweight and error-prone though, and the locking could cause problems. > >> > >> I'm wondering though, does f2fs really need try_to_fix_pino() at all, and > >> did it > >> ever really work? It never actually updates the f2fs_inode::i_name to > >> match the > >> new directory. So independently of this bug with deleted links, I don't > >> see how > >> it can possibly work as intended. > > > > Part of my humble opinion would be "update pino in rename/unlink/link... > > such ops > > instead of in fsync" (maybe it makes better sense of locking)... But > > actually I'm > > not a f2fs folk now, just curious about what the original patch resolved > > with > > these new extra igrab/iput (as I said before, I could not find some clue > > previously). > > > > Thanks, > > Gao Xiang > > > >> > >> - Eric > > > > > > _______________________________________________ > > Linux-f2fs-devel mailing list > > linux-f2fs-de...@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel > > . > >
