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SubjectRe: [PATCH] Input: mousedev - add a schedule point in mousedev_write()
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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