On Thu, Oct 04, 2018 at 12:28:56PM -0700, Eric Dumazet wrote: > On Thu, Oct 4, 2018 at 11:59 AM Dmitry Torokhov > <dmitry.torok...@gmail.com> wrote: > > > > Hi Eric, > > > > On Thu, Oct 04, 2018 at 08:47:49AM -0700, Eric Dumazet wrote: > > > syzbot was able to trigger rcu stalls by calling write() > > > with large number of bytes. > > > > > > Add a cond_resched() in the loop to avoid this. > > > > I think this simply masks a deeper issue. The code fetches characters > > from userspace in a loop, takes a lock, quickly places response in an > > output buffer, and releases interrupt. I do not see why this should > > cause stalls as we do not hold spinlock/interrupts off for extended > > period of time. > > > > Adding Paul so he can straighten me out... > > > > Well... > > write(fd, buffer, 0x7FFF0000); > > Takes between 20 seconds and 2 minutes depending on CONFIG options ....
And two minutes would get you an RCU CPU stall warning, even on distro kernels that set the stall-warning time to a full minute (as opposed to 21 seconds in mainline). > So either apply my patch, or add a limit on the max count, and > possibly break legitimate user space ? > > I dunno... I vote for Eric's patch. In fact: Reviewed-by: Paul E. McKenney <paul...@linux.ibm.com> > > > Link: https://lkml.org/lkml/2018/8/23/1106 > > > Signed-off-by: Eric Dumazet <eduma...@google.com> > > > Reported-by: syzbot+9436b02171ac0894d...@syzkaller.appspotmail.com > > > Cc: Dmitry Torokhov <dmitry.torok...@gmail.com> > > > Cc: linux-in...@vger.kernel.org > > > --- > > > drivers/input/mousedev.c | 1 + > > > 1 file changed, 1 insertion(+) > > > > > > diff --git a/drivers/input/mousedev.c b/drivers/input/mousedev.c > > > index > > > e08228061bcdd2f97aaadece31d6c83eb7539ae5..412fa71245afe26a7a8ad75705566f83633ba347 > > > 100644 > > > --- a/drivers/input/mousedev.c > > > +++ b/drivers/input/mousedev.c > > > @@ -707,6 +707,7 @@ static ssize_t mousedev_write(struct file *file, > > > const char __user *buffer, > > > mousedev_generate_response(client, c); > > > > > > spin_unlock_irq(&client->packet_lock); > > > + cond_resched(); > > > } > > > > > > kill_fasync(&client->fasync, SIGIO, POLL_IN); > > > -- > > > 2.19.0.605.g01d371f741-goog > > > > > > > Thanks. > > > > -- > > Dmitry >
