On Mon 15-06-15 09:06:11, Ted Tso wrote: > On Mon, Jun 15, 2015 at 02:33:52PM +0200, Jan Kara wrote: > > Yeah, that's nasty. Thanks for debugging this! However I think your fix > > reintroduces the original deadlock issues. do_journal_get_write_access() > > can end up blocking waiting for jbd2 thread to finish a commit while jbd2 > > thread may be blocked waiting for the page to be unlocked. > > > > After some thought I don't think the deadlock is real since > > do_journal_get_write_access() will currently only block if a buffer is > > under writeout to the journal and at that point we don't wait for page > > locks anymore. Also ext4_write_begin() does the same in data=journal mode > > and we haven't observed deadlocks so far. But still things look really > > fragile here. > > The reason why there are no deadlocks is the writeback in the commit > thread happens when the inode gets written back --- but that only > happens for data=ordered inodes, not data=journalled mode. I was a > little worried about what might happen when after the 'j' chattr > attribute gets set on an inode, and the inode was still on the ordered > flush list. > > Hmm... I think we could also maybe fix this by having > ext4_change_inode_journal_flag() force a journal commit before setting > the JOURNAL_DATA flag. If we did that, we could just avoid dropping > the page_lock in __ext4_journalled_writepage() altogether. > > What do you think? I think that fully switching lock ordering for data=journal mode back to page lock -> transaction start (which is what you effectively do when you never drop page lock in ->writepage) is rather error prone. We'd have to be careful to avoid lock inversion also for places like ->write_begin, ->releasepage, ->invalidatepage etc. For example ext4_write_begin() will currently call lock_page() with transaction started which could deadlock against journalled writepage you suggest. So effectively we'd have to completely separate aops for data=journal mode. Doable but I'm not sure it's worth it. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
