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    Subject[PATCH v3] kernel/fork: beware of __put_task_struct calling context
    Date
    Under PREEMPT_RT, __put_task_struct() indirectly acquires sleeping
    locks. Therefore, it can't be called from an non-preemptible context.

    One practical example is 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

    Instead of calling __put_task_struct() directly, we defer it using
    call_rcu(). A more natural approach would use a workqueue, but since
    in PREEMPT_RT, we can't allocate dynamic memory from atomic context,
    the code would become more complex because we would need to put the
    work_struct instance in the task_struct and initialize it when we
    allocate a new task_struct.

    Signed-off-by: Wander Lairson Costa <wander@redhat.com>
    Reported-by: Hu Chunyu <chuhu@redhat.com>
    Suggested-by: Oleg Nesterov <oleg@redhat.com>
    Suggested-by: Valentin Schneider <vschneid@redhat.com>
    Cc: Paul McKenney <paulmck@kernel.org>
    ---
    kernel/fork.c | 22 +++++++++++++++++++++-
    1 file changed, 21 insertions(+), 1 deletion(-)

    diff --git a/kernel/fork.c b/kernel/fork.c
    index 9f7fe3541897..b2d0d62c9b9d 100644
    --- a/kernel/fork.c
    +++ b/kernel/fork.c
    @@ -840,7 +840,7 @@ static inline void put_signal_struct(struct signal_struct *sig)
    free_signal_struct(sig);
    }

    -void __put_task_struct(struct task_struct *tsk)
    +static void ___put_task_struct(struct task_struct *tsk)
    {
    WARN_ON(!tsk->exit_state);
    WARN_ON(refcount_read(&tsk->usage));
    @@ -857,6 +857,26 @@ void __put_task_struct(struct task_struct *tsk)
    sched_core_free(tsk);
    free_task(tsk);
    }
    +
    +static void __delayed_put_task_struct(struct rcu_head *rhp)
    +{
    + struct task_struct *task = container_of(rhp, struct task_struct, rcu);
    +
    + ___put_task_struct(task);
    +}
    +
    +void __put_task_struct(struct task_struct *tsk)
    +{
    + if (IS_ENABLED(CONFIG_PREEMPT_RT) && (!preemptible() || !in_task()))
    + /*
    + * under PREEMPT_RT, we can't call put_task_struct
    + * in atomic context because it will indirectly
    + * acquire sleeping locks.
    + */
    + call_rcu(&tsk->rcu, __delayed_put_task_struct);
    + else
    + ___put_task_struct(tsk);
    +}
    EXPORT_SYMBOL_GPL(__put_task_struct);

    void __init __weak arch_task_cache_init(void) { }
    --
    2.39.1
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