Messages in this thread | | | Date | Thu, 4 Oct 2018 12:34:07 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] Input: mousedev - add a schedule point in mousedev_write() |
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On Thu, Oct 04, 2018 at 11:59:49AM -0700, Dmitry Torokhov 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...
If you are running a !PREEMPT kernel, then you need the cond_resched() to allow the scheduler to choose someone else to run if needed and to let RCU know that grace periods can end. Without the cond_resched(), if you stay in that loop long enough you will get excessive scheduling latencies and eventually even RCU CPU stall warning splats.
In a PREEMPT (instead of !PREEMPT) kernel, you would be right. When preemption is enabled, the scheduler can preempt and RCU can sense lack of readers from the scheduling-clock interrupt handler. Which is why cond_resched() is nothingness in a PREEMPT kernel.
But because people run !PREEMPT as well as PREEMPT kernels, if that loop can run for a long time, you need that cond_resched().
Thanx, Paul
> > Link: https://lkml.org/lkml/2018/8/23/1106 > > Signed-off-by: Eric Dumazet <edumazet@google.com> > > Reported-by: syzbot+9436b02171ac0894d33e@syzkaller.appspotmail.com > > Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com> > > Cc: linux-input@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 >
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