On Thu, Jul 26, 2018 at 11:12 AM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > On Thu, Jul 26, 2018 at 10:44 AM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote: >> On Wed, Jul 25, 2018 at 11:12 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: >>> On Tue, Jul 24, 2018 at 5:17 PM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > >>> Maybe more waits in fuse need to be interruptible? E.g. request_wait_answer? >> >> That's an interesting aspect. Making request_wait_answer always be >> killable would help with the issue you raise (killing set of processes >> taking part in deadlock should resolve deadlock), but it breaks >> another aspect of the interface. >> >> Namely that userspace filesystems expect some serialization from >> kernel when performing operations. If we allow killing of a process >> in the middle of an fs operation, then that serialization is no longer >> there, which can break the server. >> >> One solution to that is to duplicate all locking in the server >> (libfuse normally), but it would not solve the issue for legacy >> libfuse or legacy non-libfuse servers. It would also be difficult to >> test. Also it doesn't solve the problem of killing the server, as >> that alone doesn't resolve the deadlock. > > Umm, we can actually do better. Duplicate all vfs locking in the > fuse kernel implementation: when killing a task that has an > outstanding request, return immediately (which results in releasing > the VFS level lock and hence the deadlock) but hold onto our own lock > until the reply from the userspace server comes back. > > Need to think about the details; this might not be easy to do this > properly. Notably memory management locks (page->lock, mmap_sem, > etc) are notoriously tricky. Hi Miklos, Any updates on this? syzbot recently found this hang in fuse, which looks real (totally unkillable): https://syzkaller.appspot.com/bug?id=0d08132d6dac82ae63b7b8d4a9d027d30b46167d but this one still happens, and it's hard to tell if it's real or not: https://syzkaller.appspot.com/bug?id=76f8203fef423375d230f14b8f5b45617ab945e2
