On Thu, Feb 25, 2021 at 01:48:13AM +0100, Frederic Weisbecker wrote: > On Wed, Feb 24, 2021 at 04:14:25PM -0800, Paul E. McKenney wrote: > > On Wed, Feb 24, 2021 at 11:06:06PM +0100, Frederic Weisbecker wrote: > > > I managed to recollect some pieces of my brain. So keep the above but > > > let's change the point 10: > > > > > > 10. CPU 0 enqueues its second callback, this time with interrupts > > > enabled so it can wake directly ->nocb_gp_kthread. > > > It does so with calling __wake_nocb_gp() which also cancels the > > > pending timer that got queued in step 2. But that doesn't reset > > > CPU 0's ->nocb_defer_wakeup which is still set to RCU_NOCB_WAKE. > > > So CPU 0's ->nocb_defer_wakeup and CPU 0's ->nocb_timer are now > > > desynchronized. > > > > > > 11. ->nocb_gp_kthread associates the callback queued in 10 with a new > > > grace period, arrange for it to start and sleeps on it. > > > > > > 12. The grace period ends, ->nocb_gp_kthread awakens and wakes up > > > CPU 0's ->nocb_cb_kthread which invokes the callback queued in 10. > > > > > > 13. CPU 0 enqueues its third callback, this time with interrupts > > > disabled so it tries to queue a deferred wakeup. However > > > ->nocb_defer_wakeup has a stalled RCU_NOCB_WAKE value which prevents > > > the CPU 0's ->nocb_timer, that got cancelled in 10, from being armed. > > > > > > 14. CPU 0 has its pending callback and it may go unnoticed until > > > some other CPU ever wakes up ->nocb_gp_kthread or CPU 0 ever calls > > > an explicit deferred wake up caller like idle entry. > > > > > > I hope I'm not missing something this time... > > > > Thank you, that does sound plausible. I guess I can see how rcutorture > > might have missed this one! > > I must admit it requires a lot of stars to be aligned :-) It nevertheless constitutes a bug in rcutorture. Or maybe an additional challenge for the formal verification people. ;-) Thanx, Paul
