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Subject[PATCH tip/core/rcu 27/30] rcu: Make nocb_gp_wait() double-check unexpected-callback warning
From: "Paul E. McKenney" <>

Currently, nocb_gp_wait() unconditionally complains if there is a
callback not already associated with a grace period. This assumes that
either there was no such callback initially on the one hand, or that
the rcu_advance_cbs() function assigned all such callbacks to a grace
period on the other. However, in theory there are some situations that
would prevent rcu_advance_cbs() from assigning all of the callbacks.

This commit therefore checks for unassociated callbacks immediately after
rcu_advance_cbs() returns, while the corresponding rcu_node structure's
->lock is still held. If there are unassociated callbacks at that point,
the subsequent WARN_ON_ONCE() is disabled.

Signed-off-by: Paul E. McKenney <>
kernel/rcu/tree_plugin.h | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 70b3c0f..36e71c9 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1931,6 +1931,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
struct rcu_data *rdp;
struct rcu_node *rnp;
unsigned long wait_gp_seq = 0; // Suppress "use uninitialized" warning.
+ bool wasempty = false;

* Each pass through the following loop checks for CBs and for the
@@ -1970,10 +1971,13 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
rcu_seq_done(&rnp->gp_seq, cur_gp_seq))) {
raw_spin_lock_rcu_node(rnp); /* irqs disabled. */
needwake_gp = rcu_advance_cbs(rnp, rdp);
+ wasempty = rcu_segcblist_restempty(&rdp->cblist,
raw_spin_unlock_rcu_node(rnp); /* irqs disabled. */
// Need to wait on some grace period?
- WARN_ON_ONCE(!rcu_segcblist_restempty(&rdp->cblist,
+ WARN_ON_ONCE(wasempty &&
+ !rcu_segcblist_restempty(&rdp->cblist,
if (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq)) {
if (!needwait_gp ||
 \ /
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