linux-kernel - [PATCH rcu 9/9] rcu-tasks: Handle sparse cpu_possible_mask in rcu_tasks_invoke

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Date:   Mon, 18 Apr 2022 17:00:14 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        rostedt@...dmis.org, "Paul E. McKenney" <paulmck@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH rcu 9/9] rcu-tasks: Handle sparse cpu_possible_mask in rcu_tasks_invoke_cbs()

If the cpu_possible_mask is sparse (for example, if bits are set only for
CPUs 0, 4, 8, ...), then rcu_tasks_invoke_cbs() will access per-CPU data
for a CPU not in cpu_possible_mask.  It makes these accesses while doing
a workqueue-based binary search for non-empty callback lists.  Although
this search must pass through CPUs not represented in cpu_possible_mask,
it has no need to check the callback list for such CPUs.

This commit therefore changes the rcu_tasks_invoke_cbs() function's
binary search so as to only check callback lists for CPUs present in
cpu_possible_mask.

Reported-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tasks.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index fd70d86eb7cd..3925e32159b5 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -468,7 +468,7 @@ static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp, struct rcu_tasks_percpu
 		}
 	}

-	if (rcu_segcblist_empty(&rtpcp->cblist))
+	if (rcu_segcblist_empty(&rtpcp->cblist) || !cpu_possible(cpu))
 		return;
 	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
 	rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
-- 
2.31.1.189.g2e36527f23