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

[Wander Lairson Costa]

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@xxxxxxxxxx>
> > Reported-by: Hu Chunyu <chuhu@xxxxxxxxxx>
> > Cc: Paul McKenney <paulmck@xxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > ---
> >   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().

> Cheers,
> Longman
>