On 3/6/20 10:19 AM, Paul E. McKenney wrote: > On Fri, Mar 06, 2020 at 10:00:19AM -0700, Jens Axboe wrote: >> On 3/6/20 9:44 AM, Paul E. McKenney wrote: >>> 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. >> >> If the nested call_rcu() works, that seems greatly preferable to needing >> the rcu_barrier(), even if that would not be a showstopper for me. The >> nested call_rcu() is just a bit odd, but with a comment it should be OK. >> >> Incremental here I'm going to test, would just fold in of course. >> >> >> diff --git a/fs/io_uring.c b/fs/io_uring.c >> index f3218fc81943..95ba95b4d8ec 100644 >> --- a/fs/io_uring.c >> +++ b/fs/io_uring.c >> @@ -5330,7 +5330,7 @@ static void io_file_ref_kill(struct percpu_ref *ref) >> complete(&data->done); >> } >> >> -static void io_file_ref_exit_and_free(struct rcu_head *rcu) >> +static void __io_file_ref_exit_and_free(struct rcu_head *rcu) >> { >> struct fixed_file_data *data = container_of(rcu, struct fixed_file_data, >> rcu); >> @@ -5338,6 +5338,18 @@ static void io_file_ref_exit_and_free(struct rcu_head *rcu) >> kfree(data); >> } >> >> +static void io_file_ref_exit_and_free(struct rcu_head *rcu) >> +{ >> + /* >> + * We need to order our exit+free call again the potentially >> + * existing call_rcu() for switching to atomic. One way to do that >> + * is to have this rcu callback queue the final put and free, as we >> + * could otherwise a pre-existing atomic switch complete _after_ >> + * the free callback we queued. >> + */ >> + call_rcu(rcu, __io_file_ref_exit_and_free); >> +} >> + >> static int io_sqe_files_unregister(struct io_ring_ctx *ctx) >> { >> struct fixed_file_data *data = ctx->file_data; > > Looks good to me! Thanks Paul! I talked to Paul in private, but here's the final version with updated comment and attributions. https://git.kernel.dk/cgit/linux-block/commit/?h=io_uring-5.6&id=c1e2148f8ecb26863b899d402a823dab8e26efd1 -- Jens Axboe
