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Subject[PATCH v4 tip/core/rcu 30/38] rcu-tasks: Handle the running-offline idle-task special case
Date
From: "Paul E. McKenney" <paulmck@kernel.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@kernel.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
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