Re: [PATCH v6 2/3] sched/task: Add the put_task_struct_atomic_safe() function

[Waiman Long <longman@redhat.com>]

On 4/18/23 10:18, Wander Lairson Costa wrote:
> On Mon, Apr 17, 2023 at 02:51:45PM -0400, Waiman Long wrote:
>> On 4/14/23 08:55, Wander Lairson Costa wrote:
>>> Due to the possibility of indirectly acquiring sleeping locks, it is
>>> unsafe to call put_task_struct() in atomic contexts when the kernel is
>>> compiled with PREEMPT_RT.
>>>
>>> To mitigate this issue, this commit introduces
>>> put_task_struct_atomic_safe(), which schedules __put_task_struct()
>>> through call_rcu() when PREEMPT_RT is enabled. While a workqueue would
>>> be a more natural approach, we cannot allocate dynamic memory from
>>> atomic context in PREEMPT_RT, making the code more complex.
>>>
>>> This implementation ensures safe execution in atomic contexts and
>>> avoids any potential issues that may arise from using the non-atomic
>>> version.
>>>
>>> Signed-off-by: Wander Lairson Costa <wander@redhat.com>
>>> Reported-by: Hu Chunyu <chuhu@redhat.com>
>>> Cc: Paul McKenney <paulmck@kernel.org>
>>> Cc: Thomas Gleixner <tglx@linutronix.de>
>>> ---
>>>    include/linux/sched/task.h | 31 +++++++++++++++++++++++++++++++
>>>    kernel/fork.c              |  8 ++++++++
>>>    2 files changed, 39 insertions(+)
>>>
>>> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
>>> index b597b97b1f8f..5c13b83d7008 100644
>>> --- a/include/linux/sched/task.h
>>> +++ b/include/linux/sched/task.h
>>> @@ -141,6 +141,37 @@ 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() will schedule delayed_put_task_struct_rcu()
>> delayed_put_task_struct_rcu()?
>>> +			 * to be called in process context.
>>> +			 *
>>> +			 * __put_task_struct() is called called when
>> "called called"?
>>> +			 * 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.
>> Note that put_task_struct_rcu_user() isn't the only user of task->rcu.
>> delayed_free_task() in kernel/fork.c also uses it, though it is only called
>> in the error case. Still you may need to take a look to make sure that there
>> is no conflict.
>>
> delayed_free_task() is called when a process fails to start. Therefore, AFAICT,
> there is no way it can conflict with put_task_struct().

I think so too, but for completeness, you should document somewhere that 
it is a possible conflicting user.

Cheers,
Longman