Re: [PATCH v2 0/4] Fix put_task_struct() calls under PREEMPT_RT

[Valentin Schneider]

On 20/01/23 12:02, Wander Lairson Costa wrote:
> put_task_struct() decrements a usage counter and calls
> __put_task_struct() if the counter reaches zero.
>
> __put_task_struct() indirectly acquires a spinlock, which is a sleeping
> lock under PREEMPT_RT. Therefore, we can't call put_task_struct() in an
> atomic context in RT kernels.
>
> This patch series introduces put_task_struct_atomic_safe(), which defers
> the call to __put_task_struct() to a process context when compiled with
> PREEMPT_RT.
>
> It also fixes known problematic call sites.
>

Browsing around put_task_struct() callsites gives me the impression there
are more problematic call sites lurking around, which makes me wonder:
should we make the PREEMPT_RT put_task_struct() *always* be done via
call_rcu()?

The task's stack is actually always freed that way in put_task_stack(), cf.

  e540bf3162e8 ("fork: Only cache the VMAP stack in finish_task_switch()")

> Changelog:
> ==========
>
> v2:
>  * Add the put_task_struct_atomic_safe() function that is responsible for
>    handling the conditions to call put_task_struct().
>  * Replace put_task_struct() by put_task_struct_atomic_safe() in known
>    atomic call sites.
>
> Wander Lairson Costa (4):
>   sched/task: Add the put_task_struct_atomic_safe function
>   sched/deadline: fix inactive_task_timer splat
>   sched/rt: use put_task_struct_atomic_safe() to avoid potential splat
>   sched/core: use put_task_struct_atomic_safe() to avoid potential splat
>
>  include/linux/sched/task.h | 21 +++++++++++++++++++++
>  kernel/fork.c              |  8 ++++++++
>  kernel/sched/core.c        |  2 +-
>  kernel/sched/deadline.c    |  2 +-
>  kernel/sched/rt.c          |  4 ++--
>  5 files changed, 33 insertions(+), 4 deletions(-)
>
> --
> 2.39.0