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From:   Wander Lairson Costa <wander@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>,
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Subject: [PATCH v6 0/3] Introduce put_task_struct_atomic_sleep()
Date:   Fri, 14 Apr 2023 09:55:26 -0300
Message-Id: <20230414125532.14958-1-wander@redhat.com>
MIME-Version: 1.0
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Precedence: bulk

The put_task_struct() function reduces a usage counter and invokes
__put_task_struct() when the counter reaches zero.

In the case of __put_task_struct(), it indirectly acquires a spinlock,
which operates as a sleeping lock under the PREEMPT_RT configuration.
As a result, invoking put_task_struct() within an atomic context is
not feasible for real-time (RT) kernels.

To address this issue, this patch series introduces a new function
called put_task_struct_atomic_safe(). When compiled with the
PREEMPT_RT configuration, this function defers the call to
__put_task_struct() to a process context.

Additionally, the patch series rectifies known problematic call sites
to ensure smooth functioning.

Changelog
=========

v1:
* Initial implementation fixing the splat.

v2:
* Isolate the logic in its own function.
* Fix two more cases caught in review.

v3:
* Change __put_task_struct() to handle the issue internally.

v4:
* Explain why call_rcu() is safe to call from interrupt context.

v5:
* Explain why __put_task_struct() doesn't conflict with
  put_task_sruct_rcu_user.

v6:
* As per Sebastian's review, revert back the implementation of v2
  with a distinct function.
* Add a check in put_task_struct() to warning when called from a
  non-sleepable context.
* Address more call sites.

Wander Lairson Costa (3):
  sched/core: warn on call put_task_struct in invalid context
  sched/task: Add the put_task_struct_atomic_safe() function
  treewide: replace put_task_struct() witht the atomic safe version

 include/linux/sched/task.h | 45 ++++++++++++++++++++++++++++++++++++++
 kernel/events/core.c       |  6 ++---
 kernel/fork.c              |  8 +++++++
 kernel/locking/rtmutex.c   | 10 ++++-----
 kernel/sched/core.c        |  6 ++---
 kernel/sched/deadline.c    | 16 +++++++-------
 kernel/sched/rt.c          |  4 ++--
 7 files changed, 74 insertions(+), 21 deletions(-)

-- 
2.39.2