[PATCH v2 1/4] sched/task: Add the put_task_struct_atomic_safe function

[Wander Lairson Costa <wander@redhat.com>]

With PREEMPT_RT, it is unsafe to call put_task_struct() in atomic
contexts because it indirectly acquires sleeping locks.

Introduce put_task_struct_atomic_safe(), which schedules
__put_task_struct() through call_rcu() when the kernel is compiled with
PREEMPT_RT.

A more natural approach would use a workqueue, but since
in PREEMPT_RT we can't allocate dynamic memory from atomic context,
the code would become more complex because we would need to put the
work_struct instance in the task_struct and initialize it when we
allocate a new task_struct.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Cc: Paul McKenney <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
 include/linux/sched/task.h | 21 +++++++++++++++++++++
 kernel/fork.c              |  8 ++++++++
 2 files changed, 29 insertions(+)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 357e0068497c..80b4c5812563 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -127,6 +127,27 @@ static inline void put_task_struct_many(struct task_struct *t, int nr)
 
 void put_task_struct_rcu_user(struct task_struct *task);
 
+extern void __delayed_put_task_struct(struct rcu_head *rhp);
+
+static inline void put_task_struct_atomic_safe(struct task_struct *task)
+{
+	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
+		/*
+		 * Decrement the refcount explicitly to avoid unnecessarily
+		 * calling call_rcu.
+		 */
+		if (refcount_dec_and_test(&task->usage))
+			/*
+			 * under PREEMPT_RT, we can't call put_task_struct
+			 * in atomic context because it will indirectly
+			 * acquire sleeping locks.
+			 */
+			call_rcu(&task->rcu, __delayed_put_task_struct);
+	} else {
+		put_task_struct(task);
+	}
+}
+
 /* Free all architecture-specific resources held by a thread. */
 void release_thread(struct task_struct *dead_task);
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 9f7fe3541897..3d7a4e9311b3 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -859,6 +859,14 @@ void __put_task_struct(struct task_struct *tsk)
 }
 EXPORT_SYMBOL_GPL(__put_task_struct);
 
+void __delayed_put_task_struct(struct rcu_head *rhp)
+{
+	struct task_struct *task = container_of(rhp, struct task_struct, rcu);
+
+	__put_task_struct(task);
+}
+EXPORT_SYMBOL_GPL(__delayed_put_task_struct);
+
 void __init __weak arch_task_cache_init(void) { }
 
 /*
-- 
2.39.0