Commit a03ae49c authored by Neeraj Upadhyay's avatar Neeraj Upadhyay Committed by Paul E. McKenney
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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: default avatarJoel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: default avatarNeeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 9bdb5b3a

kernel/rcu/tree.c

+9 −1
Original line number Diff line number Diff line
@@ -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;