[PATCH v9 3/7] srcu: Fix invoke_rcu_callbacks() segcb length adjustment

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

With earlier patches, the negative counting of the unsegmented list
cannot be used to adjust the segmented one. To fix this, sample the
unsegmented length in advance, and use it after CB execution to adjust
the segmented list's length.

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Suggested-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/rcu/srcutree.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 0f23d20d485a..79b7081143a7 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1160,6 +1160,7 @@ static void srcu_advance_state(struct srcu_struct *ssp)
  */
 static void srcu_invoke_callbacks(struct work_struct *work)
 {
+	long len;
 	bool more;
 	struct rcu_cblist ready_cbs;
 	struct rcu_head *rhp;
@@ -1182,6 +1183,7 @@ static void srcu_invoke_callbacks(struct work_struct *work)
 	/* We are on the job!  Extract and invoke ready callbacks. */
 	sdp->srcu_cblist_invoking = true;
 	rcu_segcblist_extract_done_cbs(&sdp->srcu_cblist, &ready_cbs);
+	len = ready_cbs.len;
 	spin_unlock_irq_rcu_node(sdp);
 	rhp = rcu_cblist_dequeue(&ready_cbs);
 	for (; rhp != NULL; rhp = rcu_cblist_dequeue(&ready_cbs)) {
@@ -1190,13 +1192,14 @@ static void srcu_invoke_callbacks(struct work_struct *work)
 		rhp->func(rhp);
 		local_bh_enable();
 	}
+	WARN_ON_ONCE(ready_cbs.len);
 
 	/*
 	 * Update counts, accelerate new callbacks, and if needed,
 	 * schedule another round of callback invocation.
 	 */
 	spin_lock_irq_rcu_node(sdp);
-	rcu_segcblist_insert_count(&sdp->srcu_cblist, &ready_cbs);
+	rcu_segcblist_add_len(&sdp->srcu_cblist, -len);
 	(void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
 				       rcu_seq_snap(&ssp->srcu_gp_seq));
 	sdp->srcu_cblist_invoking = false;
-- 
2.29.1.341.ge80a0c044ae-goog