Re: [PATCH v5] debugobjects: Don't call fill_pool() in early boot hardirq context

[Ingo Molnar <mingo@kernel.org>]

* Waiman Long <longman@redhat.com> wrote:

> When booting a debug PREEMPT_RT kernel on an arm64 system with grace
> processor, a "inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage"
> lockdep warning message was reported to the console.
> 
> During early boot, interrupts are getting enabled before the scheduler
> is enabled. In this window (before SYSTEM_SCHEDULING is set) interrupts
> can fire and attempt to fill the pool from within the hardirq. This can
> lead to a deadlock the interrupt occurred while in the memory allocator.
> 
> Add a new can_fill_pool() helper and reorder the exception rule and
> forbid this scenario by excluding allocations from hardirq.
> 
> Fixes: 06e0ae988f6e ("debugobjects: Allow to refill the pool before SYSTEM_SCHEDULING")
> Co-developed-by: Waiman Long <longman@redhat.com>
> Co-developed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Co-developed-by: Thomas Gleixner <tglx@linutronix.de>
> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  lib/debugobjects.c | 46 +++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 37 insertions(+), 9 deletions(-)
> 
> diff --git a/lib/debugobjects.c b/lib/debugobjects.c
> index b18a682fe3da..6fb00e08a4e2 100644
> --- a/lib/debugobjects.c
> +++ b/lib/debugobjects.c
> @@ -720,6 +720,41 @@ static inline bool debug_objects_is_pi_blocked_on(void)
>  #endif
>  }
>  
> +static inline bool can_fill_pool(void)
> +{
> +	/*
> +	 * On !RT enabled kernels there are no restrictions and spinlock_t and
> +	 * raw_spinlock_t are the same types.
> +	 */
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> +		return true;

> -	if (!IS_ENABLED(CONFIG_PREEMPT_RT) || system_state < SYSTEM_SCHEDULING ||
> -	    (preemptible() && !debug_objects_is_pi_blocked_on())) {
> +	if (can_fill_pool()) {

Would have been nice to have this as two patches: first one
factors out the check into can_fill_pool() without changing the
logic, while the second one applies the fix to can_fill_pool().

Thanks,

	Ingo