linux-kernel - [PATCH v4 tip/core/rcu 30/38] rcu-tasks: Handle the running-offline idle-task special case

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Message-Id: <20200415181941.11653-30-paulmck@kernel.org>
Date:   Wed, 15 Apr 2020 11:19:33 -0700
From:   paulmck@...nel.org
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH v4 tip/core/rcu 30/38] rcu-tasks: Handle the running-offline idle-task special case

From: "Paul E. McKenney" <paulmck@...nel.org>

The idle task corresponding to an offline CPU can appear to be running
while that CPU is offline.  This commit therefore adds checks for this
situation, treating it as a quiescent state.  Because the tasklist scan
and the holdout-list scan now exclude CPU-hotplug operations, readers
on the CPU-hotplug paths are still waited for.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tasks.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index e8c03bd..1888a49 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -818,16 +818,20 @@ static bool trc_inspect_reader(struct task_struct *t, void *arg)
 {
 	int cpu = task_cpu(t);
 	bool in_qs = false;
+	bool ofl = cpu_is_offline(cpu);
 
 	if (task_curr(t)) {
+		WARN_ON_ONCE(ofl & !is_idle_task(t));
+
 		// If no chance of heavyweight readers, do it the hard way.
-		if (!IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
+		if (!ofl && !IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
 			return false;
 
 		// If heavyweight readers are enabled on the remote task,
 		// we can inspect its state despite its currently running.
 		// However, we cannot safely change its state.
-		if (!rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting))
+		if (!ofl && // Check for "running" idle tasks on offline CPUs.
+		    !rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting))
 			return false; // No quiescent state, do it the hard way.
 		in_qs = true;
 	} else {
-- 
2.9.5