[PATCH rcu 11/12] rcu/tree: Add comment to describe GP-done condition in fqs loop

["Paul E. McKenney" <paulmck@kernel.org>]

From: Neeraj Upadhyay <quic_neeraju@quicinc.com>

Add a comment to explain why !rcu_preempt_blocked_readers_cgp() condition
is required on root rnp node, for GP completion check in rcu_gp_fqs_loop().

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tree.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 7d8e9cb852786..cdffb3b60ee02 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2005,7 +2005,15 @@ static noinline_for_stack void rcu_gp_fqs_loop(void)
 		rcu_gp_torture_wait();
 		WRITE_ONCE(rcu_state.gp_state, RCU_GP_DOING_FQS);
 		/* Locking provides needed memory barriers. */
-		/* If grace period done, leave loop. */
+		/*
+		 * Exit the loop if the root rcu_node structure indicates that the grace period
+		 * has ended, leave the loop.  The rcu_preempt_blocked_readers_cgp(rnp) check
+		 * is required only for single-node rcu_node trees because readers blocking
+		 * the current grace period are queued only on leaf rcu_node structures.
+		 * For multi-node trees, checking the root node's ->qsmask suffices, because a
+		 * given root node's ->qsmask bit is cleared only when all CPUs and tasks from
+		 * the corresponding leaf nodes have passed through their quiescent state.
+		 */
 		if (!READ_ONCE(rnp->qsmask) &&
 		    !rcu_preempt_blocked_readers_cgp(rnp))
 			break;
-- 
2.31.1.189.g2e36527f23