Messages in this thread Patch in this message | | | Date | Thu, 9 Jun 2022 13:23:12 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] rcu/tree: Add comment to describe GP done condition in fqs loop |
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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@quicinc.com>
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
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commit bdf3a744d3ad21336a390bfcc2e41de63f193eaf Author: Neeraj Upadhyay <quic_neeraju@quicinc.com> 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@quicinc.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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;
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