[PATCH rcu/next 3/3] rcu: Fix late wakeup when flush of bypass cblist happens

[Joel Fernandes (Google)]

When the bypass cblist gets too big or its timeout has occurred, it is
flushed into the main cblist. However, the bypass timer is still running
and the behavior is that it would eventually expire and wake the GP
thread.

Since we are going to use the bypass cblist for lazy CBs, do the wakeup
soon as the flush happens. Otherwise, the lazy-timer will go off much
later and the now-non-lazy cblist CBs can get stranded for the duration
of the timer.

This is a good thing to do anyway, since it makes the behavior consistent with
behavior of other code paths where queueing something into the ->cblist makes
the GP kthread in a non-sleeping state quickly.

Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
 kernel/rcu/tree_nocb.h | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 0a5f0ef41484..04c87f250e01 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -433,8 +433,9 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
 	if ((ncbs && j != READ_ONCE(rdp->nocb_bypass_first)) ||
 	    ncbs >= qhimark) {
 		rcu_nocb_lock(rdp);
+		*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
+
 		if (!rcu_nocb_flush_bypass(rdp, rhp, j)) {
-			*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
 			if (*was_alldone)
 				trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
 						    TPS("FirstQ"));
@@ -447,7 +448,12 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
 			rcu_advance_cbs_nowake(rdp->mynode, rdp);
 			rdp->nocb_gp_adv_time = j;
 		}
-		rcu_nocb_unlock_irqrestore(rdp, flags);
+
+		// The flush succeeded and we moved CBs into the regular list.
+		// Don't wait for the wake up timer as it may be too far ahead.
+		// Wake up the GP thread now instead, if the cblist was empty.
+		__call_rcu_nocb_wake(rdp, *was_alldone, flags);
+
 		return true; // Callback already enqueued.
 	}
 
-- 
2.37.2.789.g6183377224-goog