Commit d0a13c39 authored Jun 14, 2023 by Wander Lairson Costa Committed by Greg Kroah-Hartman Sep 23, 2023

kernel/fork: beware of __put_task_struct() calling context



[ Upstream commit d243b34459cea30cfe5f3a9b2feb44e7daff9938 ]

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
   mark_lock+0x1e7/0x400
   mark_usage+0x11d/0x140
   __lock_acquire+0x30d/0x930
   lock_acquire.part.0+0x9c/0x210
   rt_spin_lock+0x27/0xe0
   refill_obj_stock+0x3d/0x3a0
   kmem_cache_free+0x357/0x560
   inactive_task_timer+0x1ad/0x340
   __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+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.

The issue is reproducible with stress-ng:

  while true; do
      stress-ng --sched deadline --sched-period 1000000000 \
	      --sched-runtime 800000000 --sched-deadline \
	      1000000000 --mmapfork 23 -t 20
  done

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>
Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230614122323.37957-2-wander@redhat.com


Signed-off-by: Sasha Levin <sashal@kernel.org>

parent 3bf4463e

include/linux/sched/task.h

+27 −1

Original line number	Diff line number	Diff line
		@@ -113,10 +113,36 @@ 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);
		}

kernel/fork.c

+8 −0

Original line number	Diff line number	Diff line
		@@ -749,6 +749,14 @@ 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) { }

		/*