On Thu, Oct 04, 2018 at 12:34:07PM -0700, Paul E. McKenney wrote: > 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(). OK, I see. I'll apply the patch then. I think evdev.c needs similar treatment as it will keep looping while there is data... Thanks. -- Dmitry
