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    Subject[PATCH v7 0/3] Introduce put_task_struct_atomic_sleep()
    Date
    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.

    One practical example is a splat inside inactive_task_timer(), which is
    called in a interrupt context:

    CPU: 1 PID: 2848 Comm: life Kdump: loaded Tainted: G W ---------
    Hardware name: HP ProLiant DL388p Gen8, BIOS P70 07/15/2012
    Call Trace:
    dump_stack_lvl+0x57/0x7d
    mark_lock_irq.cold+0x33/0xba
    ? stack_trace_save+0x4b/0x70
    ? save_trace+0x55/0x150
    mark_lock+0x1e7/0x400
    mark_usage+0x11d/0x140
    __lock_acquire+0x30d/0x930
    lock_acquire.part.0+0x9c/0x210
    ? refill_obj_stock+0x3d/0x3a0
    ? rcu_read_lock_sched_held+0x3f/0x70
    ? trace_lock_acquire+0x38/0x140
    ? lock_acquire+0x30/0x80
    ? refill_obj_stock+0x3d/0x3a0
    rt_spin_lock+0x27/0xe0
    ? refill_obj_stock+0x3d/0x3a0
    refill_obj_stock+0x3d/0x3a0
    ? inactive_task_timer+0x1ad/0x340
    kmem_cache_free+0x357/0x560
    inactive_task_timer+0x1ad/0x340
    ? switched_from_dl+0x2d0/0x2d0
    __run_hrtimer+0x8a/0x1a0
    __hrtimer_run_queues+0x91/0x130
    hrtimer_interrupt+0x10f/0x220
    __sysvec_apic_timer_interrupt+0x7b/0xd0
    sysvec_apic_timer_interrupt+0x4f/0xd0
    ? asm_sysvec_apic_timer_interrupt+0xa/0x20
    asm_sysvec_apic_timer_interrupt+0x12/0x20
    RIP: 0033:0x7fff196bf6f5

    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.

    v7:
    * Fix typos.
    * Add an explanation why the new function doesn't conflict with
    delayed_free_task().

    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() with the atomic safe version

    include/linux/sched/task.h | 49 ++++++++++++++++++++++++++++++++++++++
    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, 78 insertions(+), 21 deletions(-)

    --
    2.40.0

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