On Mon, Mar 01, 2021 at 05:48:29PM -0800, Paul E. McKenney wrote: > On Wed, Feb 24, 2021 at 11:06:06PM +0100, Frederic Weisbecker wrote: > > On Wed, Feb 24, 2021 at 10:37:09AM -0800, Paul E. McKenney wrote: > > > On Tue, Feb 23, 2021 at 01:09:59AM +0100, Frederic Weisbecker wrote: > > > > Two situations can cause a missed nocb timer rearm: > > > > > > > > 1) rdp(CPU A) queues its nocb timer. The grace period elapses before > > > > the timer get a chance to fire. The nocb_gp kthread is awaken by > > > > rdp(CPU B). The nocb_cb kthread for rdp(CPU A) is awaken and > > > > process the callbacks, again before the nocb_timer for CPU A get a > > > > chance to fire. rdp(CPU A) queues a callback and wakes up nocb_gp > > > > kthread, cancelling the pending nocb_timer without resetting the > > > > corresponding nocb_defer_wakeup. > > > > > > As discussed offlist, expanding the above scenario results in this > > > sequence of steps: > > I renumbered the CPUs, since the ->nocb_gp_kthread would normally be > associated with CPU 0. If the first CPU to enqueue a callback was also > CPU 0, nocb_gp_wait() might clear that CPU's ->nocb_defer_wakeup, which > would prevent this scenario from playing out. (But admittedly only if > some other CPU handled by this same ->nocb_gp_kthread used its bypass.) Ok good point. > > > > 1. There are no callbacks queued for any CPU covered by CPU 0-2's > > > ->nocb_gp_kthread. > > And ->nocb_gp_kthread is associated with CPU 0. > > > > 2. CPU 1 enqueues its first callback with interrupts disabled, and > > > thus must defer awakening its ->nocb_gp_kthread. It therefore > > > queues its rcu_data structure's ->nocb_timer. > > At this point, CPU 1's rdp->nocb_defer_wakeup is RCU_NOCB_WAKE. Right. > > > 7. The grace period ends, so rcu_gp_kthread awakens the > > > ->nocb_gp_kthread, which in turn awakens both CPU 1's and > > > CPU 2's ->nocb_cb_kthread. > > And then ->nocb_cb_kthread sleeps waiting for more callbacks. Yep > > I managed to recollect some pieces of my brain. So keep the above but > > let's change the point 10: > > > > 10. CPU 1 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 > > wake_nocb_gp() in current -rcu, correct? Heh, right. > > > So far so good, but why isn't the timer still queued from back in step 2? > > > What am I missing here? Either way, could you please update the commit > > > logs to tell the full story? At some later time, you might be very > > > happy that you did. ;-) > > > > > > > 2) The "nocb_bypass_timer" ends up calling wake_nocb_gp() which deletes > > > > the pending "nocb_timer" (note they are not the same timers) for the > > > > given rdp without resetting the matching state stored in nocb_defer > > > > wakeup. > > Would like to similarly expand this one, or would you prefer to rest your > case on Case 1) above? I was about to say that we can skip that one, the changelog will already be big enough but the "Fixes:" tag refers to the second scenario, since it's the oldest vulnerable commit AFAICS. > > > > Fixes: d1b222c6be1f (rcu/nocb: Add bypass callback queueing) Thanks.
