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

[Neeraj Upadhyay]

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().

Signed-off-by: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
---
 kernel/rcu/tree.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a93c5f4d7e09..9cd1ba512fdc 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2083,7 +2083,17 @@ 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. */
+		/*
+		 * If grace period done, leave loop. rcu_preempt_blocked_readers_cgp(rnp)
+		 * check is required for the case where we only have single node in the
+		 * rcu_node tree; so, root rcu node is also the leaf node, where readers
+		 * blocking current gp are queued. For multi-node tree, checking ->qsmask
+		 * on the root node is sufficient, as root rcu node's ->qsmask is only
+		 * cleared, when all leaf rcu nodes have propagated their quiescent
+		 * state to their parent node, which happens only after both ->qsmask
+		 * and rcu_preempt_blocked_readers_cgp(rnp_leaf) are cleared for those
+		 * leaf nodes.
+		 */
 		if (!READ_ONCE(rnp->qsmask) &&
 		    !rcu_preempt_blocked_readers_cgp(rnp))
 			break;
-- 
2.17.1