rcu/tree: Add comment to describe GP-done condition in fqs loop

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>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a23877a..7c22318 100644 (file)
--- 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;