On Fri, Mar 05, 2021 at 02:10:09PM +0100, Alexander Lochmann wrote: > Hi folks, > > I've stumbled across an interesting locking scheme. It's related to struct > inode, more precisely it is an mqueue inode. > Our results show that inode:mqueue.i_opflags is read with i_rwsem being > hold. > In d_flags_for_inode, and do_inode_permission the i_lock is used to read and > write i_opflags. > Is this a real locking scheme? Is a lock needed to access i_opflags at all? > What is the magic behind this contradiction? > > I've put the report of the counterexamples on our webserver: > https://ess.cs.tu-dortmund.de/lockdoc-bugs/cex-inode-mqueue.html. > It contains the stacktraces leading to those accesses, and the locks that > were actually held. 1) I don't see where i_opflags is being read in ipc/mqueue.c at all, either with or without i_rwsem. 2) I'm not sure what this has to do with ext4? - Ted
