Re: [PATCH v7 0/3] Introduce put_task_struct_atomic_sleep()

[Valentin Schneider]

On 25/04/23 08:43, Wander Lairson Costa wrote:
> 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.
>

It took me a bit of time to grok the put_task_struct_rcu_user() vs
delayed_free_task() vs put_task_struct_atomic_safe() situation, but other
than that the patches LGTM.

Reviewed-by: Valentin Schneider <vschneid@xxxxxxxxxx>