In message <20080424173013.GA5882@xxxxxxxxxxxxxxxxxx>, Al Viro writes: > On Thu, Apr 24, 2008 at 01:25:58PM -0400, Erez Zadok wrote: > > Al, any near-term plans for sb-level "want write" locking as we discussed > > briefly at LSF? Being able to do so for copyup in unionfs will hopefully > > allow me to prevent concurrent topology changes. > > That's pretty close to the top. > > > the two directories that need locking. This rename-locking protocol appears > > to be a special case of the sb-level "want write" idea, right? > > Not really - that one doesn't provide any exclusion between writers and > that's what we want from rename/rename. When you say "that one", do you mean the sb-level idea? If the sb-level "want write" not provide exclusion among writers, then how can I prevent lower topology changes while copyup takes place? If your sb-level "want write" idea won't provide that exclusion at the sb level, then how about we elevate the s_vfs_rename_mutex into a generic s_vfs_multi_dir_change_mutex of sorts, which can be used by rename as well as copyup? > Take a look at Documentation/filesystems/directory-locking for details > of locking scheme... OK. lock_rename() at most locks one "parent" and one "child" inodes. So what happens to all the ones in b/t? Suppose I have a path /a/b/c/d/e/f and someone wants to mv /a/b/c/d/e to /a/b/; in that case lock_rename at least grabs the mutex on /a/b, right? Can someone go in the middle and try to muck with the "c/d/" parts in between? >From what I gathered, lock_rename won't be enough for me to prevent topology changes during copyup; I'd really need to lock an entire arbitrarily deep path of inodes. That just seems so complex and prone to bugs, that it might just be easier to have a single sb-level mutex for these complex multi-directory operations. I'm not sure how much performance hit this'd be, thou, and if there's a more efficient way to do so. If you think lock_rename will be sufficient for me to deal with copyups vs. topology changes, then I'll start using it. Thanks, Erez. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
