On Fri, Mar 06, 2020 at 04:36:20PM +0100, Jann Horn wrote: > On Fri, Mar 6, 2020 at 4:34 PM Jens Axboe <axboe@xxxxxxxxx> wrote: > > On 3/6/20 7:57 AM, Jann Horn wrote: > > > +paulmck > > > > > > On Wed, Mar 4, 2020 at 3:40 PM Jens Axboe <axboe@xxxxxxxxx> wrote: > > >> On 3/4/20 12:59 AM, Dmitry Vyukov wrote: > > >>> On Fri, Feb 7, 2020 at 9:14 AM syzbot > > >>> <syzbot+e017e49c39ab484ac87a@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > > >>>> > > >>>> Hello, > > >>>> > > >>>> syzbot found the following crash on: > > >>>> > > >>>> HEAD commit: 4c7d00cc Merge tag 'pwm/for-5.6-rc1' of git://git.kernel.o.. > > >>>> git tree: upstream > > >>>> console output: https://syzkaller.appspot.com/x/log.txt?x=12fec785e00000 > > >>>> kernel config: https://syzkaller.appspot.com/x/.config?x=e162021ddededa72 > > >>>> dashboard link: https://syzkaller.appspot.com/bug?extid=e017e49c39ab484ac87a > > >>>> compiler: clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81) > > >>>> > > >>>> Unfortunately, I don't have any reproducer for this crash yet. > > >>>> > > >>>> IMPORTANT: if you fix the bug, please add the following tag to the commit: > > >>>> Reported-by: syzbot+e017e49c39ab484ac87a@xxxxxxxxxxxxxxxxxxxxxxxxx > > >>> > > >>> +io_uring maintainers > > >>> > > >>> Here is a repro: > > >>> https://gist.githubusercontent.com/dvyukov/6b340beab6483a036f4186e7378882ce/raw/cd1922185516453c201df8eded1d4b006a6d6a3a/gistfile1.txt > > >> > > >> I've queued up a fix for this: > > >> > > >> https://git.kernel.dk/cgit/linux-block/commit/?h=io_uring-5.6&id=9875fe3dc4b8cff1f1b440fb925054a5124403c3 > > > > > > I believe that this fix relies on call_rcu() having FIFO ordering; but > > > <https://www.kernel.org/doc/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html#Callback%20Registry> > > > says: > > > > > > | call_rcu() normally acts only on CPU-local state[...] It simply > > > enqueues the rcu_head structure on a per-CPU list, Indeed. For but one example, if there was a CPU-to-CPU migration between the two call_rcu() invocations, it would not be at all surprising for the two callbacks to execute out of order. > > > Is this fix really correct? > > > > That's a good point, there's a potentially stronger guarantee we need > > here that isn't "nobody is inside an RCU critical section", but rather > > that we're depending on a previous call_rcu() to have happened. Hence I > > think you are right - it'll shrink the window drastically, since the > > previous callback is already queued up, but it's not a full close. > > > > Hmm... > > You could potentially hack up the semantics you want by doing a > call_rcu() whose callback does another call_rcu(), or something like > that - but I'd like to hear paulmck's opinion on this first. That would work! Or, alternatively, do an rcu_barrier() between the two calls to call_rcu(), assuming that the use case can tolerate rcu_barrier() overhead and latency. Thanx, Paul
