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

[Paul E. McKenney]

On Mon, Apr 24, 2023 at 05:34:29PM -0300, Wander Lairson Costa wrote:
> On Mon, Apr 24, 2023 at 3:52 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > On Mon, Apr 24, 2023 at 03:43:09PM -0300, Wander Lairson Costa wrote:
> > > On Mon, Apr 24, 2023 at 3:09 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > > > On Fri, Apr 14, 2023 at 09:55:28AM -0300, 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()
> > > > > +                      * to be called in process context.
> > > > > +                      *
> > > > > +                      * __put_task_struct() is called 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.
> > > > > +                      */
> > > > > +                     call_rcu(&task->rcu, __delayed_put_task_struct);
> > > >
> > > > This will invoke __delayed_put_task_struct() with softirqs disabled.
> > > > Or do softirq-disabled contexts count as non-atomic in PREEMPT_RT?
> > >
> > > softirqs execute in thread context in PREEMPT_RT. We are good here.
> >
> > So the sleeping lock is a spinlock rather than (say) a mutex?
> 
> Yes, under PREEMPT_RT, spinlocks are implemented in terms of rtmutex.

Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

                                                        Thanx, Paul

> > > > > +     } 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 0c92f224c68c..9884794fe4b8 100644
> > > > > --- a/kernel/fork.c
> > > > > +++ b/kernel/fork.c
> > > > > @@ -854,6 +854,14 @@ 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) { }
> > > > >
> > > > >  /*
> > > > > --
> > > > > 2.39.2
> > > > >
> > > >
> > >
> >
>