On Mon, Dec 16, 2019 at 1:58 PM Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > > On Thu, Dec 12, 2019 at 4:43 PM Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > > It's the same old story that was fixed in commit: > > 6d0a8a90a5bb ovl: take lower dir inode mutex outside upper sb_writers lock > > > > The lower overlay inode mutex is taken inside ovl_llseek() while upper fs > > sb_writers is held since ovl_maybe_copy_up() of nested overlay. > > > > Since the lower overlay uses same real fs as nested overlay upper, > > this could really deadlock if the lower overlay inode is being modified > > (took inode mutex and trying to take real fs sb_writers). > > > > Not a very common case, but still a possible deadlock. > > > > The only way to avoid this deadlock is probably a bit too hacky for your taste: > > > > /* Skip copy hole optimization for nested overlay */ > > if (old->mnt->mnt_sb->s_stack_depth) > > skip_hole = false; > > > > The other way is to use ovl_inode_lock() in ovl_llseek(). > > > > Have any preference? Something else? > > > > Should we maybe use ovl_inode_lock() also in ovl_write_iter() and > > ovl_ioctl_set_flags()? In all those cases, we are not protecting the overlay > > inode members, but the real inode members from concurrent modification > > through overlay. > Using ovl_inode_lock() in ovl_write_iter() and ovl_ioctl_set_flags() is not as simple as in ovl_llseek(). And it is less important because those call can not be made on a lower overlay. So I'll send patches to convert ovl_llseek() ovl_dir_llseek() and ovl_dir_fsync() to use ovl_inode_lock(), which seems simple and passes the tests. Thanks, Amir.
