rcu: Enable RCU callbacks to benefit from expedited grace periods

Currently, RCU callbacks only track normal grace-period sequence
numbers.  This means callbacks must wait for normal grace periods to
complete even when expedited grace periods have already elapsed.

Use the full struct rcu_gp_seq (which tracks both the normal and
expedited grace-period sequences) throughout the callback
infrastructure.

rcu_segcblist_advance() now checks both normal and expedited GP
completion via poll_state_synchronize_rcu_full(), and becomes
parameterless since it reads the grace-period state internally.
rcu_segcblist_accelerate() stores the full state (both sequences)
instead of just the normal one.  rcu_accelerate_cbs() and
rcu_accelerate_cbs_unlocked() use get_state_synchronize_rcu_full() to
capture both sequences, and the NOCB advance checks use
poll_state_synchronize_rcu_full() instead of comparing only the normal
sequence.

srcu_segcblist_advance() becomes a standalone implementation because it
compares SRCU sequences directly and cannot use
poll_state_synchronize_rcu_full(), which reads RCU-specific globals.
srcu_segcblist_accelerate() sets the ->exp field to
RCU_GET_STATE_NOT_TRACKED so that poll_state_synchronize_rcu_full()
compares only ->norm and ignores ->exp.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 3 insertions, 6 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 640ebe055a24..f63ce75c5b4f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1159,7 +1159,7 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
 	 * accelerating callback invocation to an earlier grace-period
 	 * number.
 	 */
-	gs.norm = rcu_seq_snap(&rcu_state.gp_seq);
+	get_state_synchronize_rcu_full(&gs);
 	if (rcu_segcblist_accelerate(&rdp->cblist, &gs))
 		ret = rcu_start_this_gp(rnp, rdp, gs.norm);
 
@@ -1188,7 +1188,7 @@ static void rcu_accelerate_cbs_unlocked(struct rcu_node *rnp,
 	bool needwake;
 
 	rcu_lockdep_assert_cblist_protected(rdp);
-	gs.norm = rcu_seq_snap(&rcu_state.gp_seq);
+	get_state_synchronize_rcu_full(&gs);
 	if (!READ_ONCE(rdp->gpwrap) && ULONG_CMP_GE(rdp->gp_seq_needed, gs.norm)) {
 		/* Old request still live, so mark recent callbacks. */
 		(void)rcu_segcblist_accelerate(&rdp->cblist, &gs);
@@ -1213,8 +1213,6 @@ static void rcu_accelerate_cbs_unlocked(struct rcu_node *rnp,
  */
 static bool rcu_advance_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
 {
-	struct rcu_gp_seq gs;
-
 	rcu_lockdep_assert_cblist_protected(rdp);
 	raw_lockdep_assert_held_rcu_node(rnp);
 
@@ -1226,8 +1224,7 @@ static bool rcu_advance_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
 	 * Find all callbacks whose ->gp_seq numbers indicate that they
 	 * are ready to invoke, and put them into the RCU_DONE_TAIL sublist.
 	 */
-	gs.norm = rnp->gp_seq;
-	rcu_segcblist_advance(&rdp->cblist, &gs);
+	rcu_segcblist_advance(&rdp->cblist);
 
 	/* Classify any remaining callbacks. */
 	return rcu_accelerate_cbs(rnp, rdp);