[PATCH v9] kernel/fork: beware of __put_task_struct calling context

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

Under PREEMPT_RT, __put_task_struct() indirectly acquires sleeping
locks. Therefore, it can't be called from an non-preemptible context.

One practical example is splat inside inactive_task_timer(), which is
called in a interrupt context:

CPU: 1 PID: 2848 Comm: life Kdump: loaded Tainted: G W ---------
 Hardware name: HP ProLiant DL388p Gen8, BIOS P70 07/15/2012
 Call Trace:
 dump_stack_lvl+0x57/0x7d
 mark_lock_irq.cold+0x33/0xba
 ? stack_trace_save+0x4b/0x70
 ? save_trace+0x55/0x150
 mark_lock+0x1e7/0x400
 mark_usage+0x11d/0x140
 __lock_acquire+0x30d/0x930
 lock_acquire.part.0+0x9c/0x210
 ? refill_obj_stock+0x3d/0x3a0
 ? rcu_read_lock_sched_held+0x3f/0x70
 ? trace_lock_acquire+0x38/0x140
 ? lock_acquire+0x30/0x80
 ? refill_obj_stock+0x3d/0x3a0
 rt_spin_lock+0x27/0xe0
 ? refill_obj_stock+0x3d/0x3a0
 refill_obj_stock+0x3d/0x3a0
 ? inactive_task_timer+0x1ad/0x340
 kmem_cache_free+0x357/0x560
 inactive_task_timer+0x1ad/0x340
 ? switched_from_dl+0x2d0/0x2d0
 __run_hrtimer+0x8a/0x1a0
 __hrtimer_run_queues+0x91/0x130
 hrtimer_interrupt+0x10f/0x220
 __sysvec_apic_timer_interrupt+0x7b/0xd0
 sysvec_apic_timer_interrupt+0x4f/0xd0
 ? asm_sysvec_apic_timer_interrupt+0xa/0x20
 asm_sysvec_apic_timer_interrupt+0x12/0x20
 RIP: 0033:0x7fff196bf6f5

Instead of calling __put_task_struct() directly, we defer it using
call_rcu(). 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.

Changelog
=========

v1:
* Initial implementation fixing the splat.

v2:
* Isolate the logic in its own function.
* Fix two more cases caught in review.

v3:
* Change __put_task_struct() to handle the issue internally.

v4:
* Explain why call_rcu() is safe to call from interrupt context.

v5:
* Explain why __put_task_struct() doesn't conflict with
  put_task_sruct_rcu_user.

v6:
* As per Sebastian's review, revert back the implementation of v2
  with a distinct function.
* Add a check in put_task_struct() to warning when called from a
  non-sleepable context.
* Address more call sites.

v7:
* Fix typos.
* Add an explanation why the new function doesn't conflict with
  delayed_free_task().

v8:
* Bring back v5.
* Fix coding style.

v9:
* Reorganize to not need ___put_task_struct() by Oleg's suggestion.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Reported-by: Hu Chunyu <chuhu@redhat.com>
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Suggested-by: Valentin Schneider <vschneid@redhat.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Paul McKenney <paulmck@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Luis Goncalves <lgoncalv@redhat.com>
---
 include/linux/sched/task.h | 28 +++++++++++++++++++++++++++-
 kernel/fork.c              |  8 ++++++++
 2 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index d6c48163c6de..9bcb9535d4e1 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -112,10 +112,36 @@ static inline struct task_struct *get_task_struct(struct task_struct *t)
 }
 
 extern void __put_task_struct(struct task_struct *t);
+extern void __put_task_struct_rcu_cb(struct rcu_head *rhp);
 
 static inline void put_task_struct(struct task_struct *t)
 {
-	if (refcount_dec_and_test(&t->usage))
+	if (!refcount_dec_and_test(&t->usage))
+		return;
+
+	/*
+	 * under PREEMPT_RT, we can't call put_task_struct
+	 * in atomic context because it will indirectly
+	 * acquire sleeping locks.
+	 *
+	 * call_rcu() will schedule delayed_put_task_struct_rcu()
+	 * to be called in process context.
+	 *
+	 * __put_task_struct() is called when
+	 * refcount_dec_and_test(&t->usage) succeeds.
+	 *
+	 * This means that it can't "conflict" with
+	 * put_task_struct_rcu_user() which abuses ->rcu the same
+	 * way; rcu_users has a reference so task->usage can't be
+	 * zero after rcu_users 1 -> 0 transition.
+	 *
+	 * delayed_free_task() also uses ->rcu, but it is only called
+	 * when it fails to fork a process. Therefore, there is no
+	 * way it can conflict with put_task_struct().
+	 */
+	if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
+		call_rcu(&t->rcu, __put_task_struct_rcu_cb);
+	else
 		__put_task_struct(t);
 }
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 08969f5aa38d..fd3bb4a554c4 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -846,6 +846,14 @@ void __put_task_struct(struct task_struct *tsk)
 }
 EXPORT_SYMBOL_GPL(__put_task_struct);
 
+void __put_task_struct_rcu_cb(struct rcu_head *rhp)
+{
+	struct task_struct *task = container_of(rhp, struct task_struct, rcu);
+
+	__put_task_struct(task);
+}
+EXPORT_SYMBOL_GPL(__put_task_struct_rcu_cb);
+
 void __init __weak arch_task_cache_init(void) { }
 
 /*
-- 
2.40.1