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

[Paul E. McKenney]

On Thu, Jun 09, 2022 at 12:43:40PM +0530, Neeraj Upadhyay wrote:
> 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>

Thank you, Neeraj!  As usual, I could not resist the urge to wordsmith
as shown below.  Could you please check to see if I messed something up?

							Thanx, Paul

------------------------------------------------------------------------

commit bdf3a744d3ad21336a390bfcc2e41de63f193eaf
Author: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
Date:   Thu Jun 9 12:43:40 2022 +0530

    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().
    
    Signed-off-by: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a93c5f4d7e092..9a941e7ee6109 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2083,7 +2083,15 @@ 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 node have passed through their quiescent state.
+		 */
 		if (!READ_ONCE(rnp->qsmask) &&
 		    !rcu_preempt_blocked_readers_cgp(rnp))
 			break;