From: Frederic Weisbecker frederic@kernel.org
This sequence of events can lead to a failure to requeue a CPU's ->nocb_timer:
1. There are no callbacks queued for any CPU covered by CPU 0-2's ->nocb_gp_kthread. Note that ->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.
3. CPU 2, which shares the same ->nocb_gp_kthread, also enqueues a callback, but with interrupts enabled, allowing it to directly awaken the ->nocb_gp_kthread.
4. The newly awakened ->nocb_gp_kthread associates both CPU 1's and CPU 2's callbacks with a future grace period and arranges for that grace period to be started.
5. This ->nocb_gp_kthread goes to sleep waiting for the end of this future grace period.
6. This grace period elapses before the CPU 1's timer fires. This is normally improbably given that the timer is set for only one jiffy, but timers can be delayed. Besides, it is possible that kernel was built with CONFIG_RCU_STRICT_GRACE_PERIOD=y.
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. Then ->nocb_gb_kthread sleeps waiting for more newly queued callbacks.
8. CPU 1's ->nocb_cb_kthread invokes its callback, then sleeps waiting for more invocable callbacks.
9. Note that neither kthread updated any ->nocb_timer state, so CPU 1's ->nocb_defer_wakeup is still set to RCU_NOCB_WAKE.
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 pending timer that got queued in step 2. But that doesn't reset CPU 1's ->nocb_defer_wakeup which is still set to RCU_NOCB_WAKE. So CPU 1's ->nocb_defer_wakeup and its ->nocb_timer are now desynchronized.
11. ->nocb_gp_kthread associates the callback queued in 10 with a new grace period, arranges for that grace period to start and sleeps waiting for it to complete.
12. The grace period ends, rcu_gp_kthread awakens ->nocb_gp_kthread, which in turn wakes up CPU 1's ->nocb_cb_kthread which then invokes the callback queued in 10.
13. CPU 1 enqueues its third callback, this time with interrupts disabled so it must queue a timer for a deferred wakeup. However the value of its ->nocb_defer_wakeup is RCU_NOCB_WAKE which incorrectly indicates that a timer is already queued. Instead, CPU 1's ->nocb_timer was cancelled in 10. CPU 1 therefore fails to queue the ->nocb_timer.
14. CPU 1 has its pending callback and it may go unnoticed until some other CPU ever wakes up ->nocb_gp_kthread or CPU 1 ever calls an explicit deferred wakeup, for example, during idle entry.
This commit fixes this bug by resetting rdp->nocb_defer_wakeup everytime we delete the ->nocb_timer.
It is quite possible that there is a similar scenario involving ->nocb_bypass_timer and ->nocb_defer_wakeup. However, despite some effort from several people, a failure scenario has not yet been located. However, that by no means guarantees that no such scenario exists. Finding a failure scenario is left as an exercise for the reader, and the "Fixes:" tag below relates to ->nocb_bypass_timer instead of ->nocb_timer.
Fixes: d1b222c6be1f (rcu/nocb: Add bypass callback queueing) Cc: stable@vger.kernel.org Cc: Josh Triplett josh@joshtriplett.org Cc: Lai Jiangshan jiangshanlai@gmail.com Cc: Joel Fernandes joel@joelfernandes.org Cc: Boqun Feng boqun.feng@gmail.com Reviewed-by: Neeraj Upadhyay neeraju@codeaurora.org Signed-off-by: Frederic Weisbecker frederic@kernel.org Signed-off-by: Paul E. McKenney paulmck@kernel.org --- kernel/rcu/tree_plugin.h | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index a1a17ad..e392bd1 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -1708,7 +1708,11 @@ static bool wake_nocb_gp(struct rcu_data *rdp, bool force, rcu_nocb_unlock_irqrestore(rdp, flags); return false; } - del_timer(&rdp->nocb_timer); + + if (READ_ONCE(rdp->nocb_defer_wakeup) > RCU_NOCB_WAKE_NOT) { + WRITE_ONCE(rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT); + del_timer(&rdp->nocb_timer); + } rcu_nocb_unlock_irqrestore(rdp, flags); raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags); if (force || READ_ONCE(rdp_gp->nocb_gp_sleep)) { @@ -2335,7 +2339,6 @@ static bool do_nocb_deferred_wakeup_common(struct rcu_data *rdp) return false; } ndw = READ_ONCE(rdp->nocb_defer_wakeup); - WRITE_ONCE(rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT); ret = wake_nocb_gp(rdp, ndw == RCU_NOCB_WAKE_FORCE, flags); trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DeferredWake"));