Theodore Y. Ts'o wrote: > On Tue, May 08, 2018 at 08:05:12PM +0900, Tetsuo Handa wrote: > > > > So, it is time to think how to solve this race condition, as well as how to solve > > lockdep's deadlock warning (and I guess that syzbot is actually hitting deadlocks). > > An approach which serializes loop operations using global lock was proposed at > > https://groups.google.com/d/msg/syzkaller-bugs/2Rw8-OM6IbM/PzdobV8kAgAJ . > > Please respond... > > I'm looking at your patch which you proposed on this, and the locking > architecture still looks way too complex. Things like > loop_mutex_owner, and all of the infrastructure around > lo->ioctl_in_progress should be removed, if at all possible. The patch in the above link no longer uses "lo->ioctl_in_progress". You looked at previous version rather than current version. > > I believe it should be possible to do things with a single global > mutex, some code refactoring, and some unlocked versions of some of > the functions. The patch in the above link uses single global mutex "loop_mutex". > > Again, this requires root, and it requires someone deliberately trying > to induce a race. So "it's time" is not necessarily the priority I > would set for this item. But if we are going to fix it, let's fix it > right, and not make the code more complex and less maintainable, all > in the name of trying to make a rare, not-likely-to-happen-in-real-life > syzbot reported problem to go away. While NULL pointer dereference would be rare, deadlocks might not be rare enough to postpone the patch. Deadlocks can cause pile of false-positive hung task reports and can prevent syzbot from finding other bugs. That's why I say "it is time to think".
