On Fri, Mar 22, 2013 at 06:11:11PM +0000, Al Viro wrote: > On Sat, Mar 23, 2013 at 02:37:55AM +0900, J. R. Okajima wrote: > > > > David Howells: > > > Now, looking at __sb_start_write(), I'm not entirely sure how the deadlock > > > might operate, so it's possible that this is a false alarm. Maybe Jan Kara can > > > illuminate further, so I've added him to the cc list. > > > > It is related to the design of UnionMount, isn't it? > > UnionMount is not a filesystem and doen't have its own superblock. > > If it was a fs, then > > - vfs_truncate() acquires sb_writers for the unioning-fs. > > - the unioning-fs may call vfs_truncate() again for the underlying fs. > > - this time, sb_writers is for the underlying fs which is a different > > sb_writers object from the already acquired one. > > So there would be no deadlock. > > Doesn't help the situation with copyup - witness overlayfs stepping into the > same deadlock on copyup. It wants ->i_mutex held on directory in upper layer > and it tries to write to file it has created in there. The problem is > with the upper layer superblock getting frozen; having a separate one for > union is irrelevant. Let me check how aufs does... Aha. Your > au_do_copy_file() ends up calling vfs_write() on the file opened in > upper layer. And AFAICS it's called with ->i_mutex held on the directory > in upper layer, so you've got the same deadlock, sorry. The scenario, BTW, looks so: process A does sb_start_write() (on your upper layer) process B tries to freeze said upper layer and blocks, waiting for A to finish process C grabs ->i_mutex in your upper layer process C does vfs_write(), which blocks, since there's a pending attempt to freeze process A tries to grab ->i_mutex held by C and blocks -- 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
