Re: [PATCH PREEMPT_RT] i915: fix PREEMPT_RT locking splats

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2021-08-23 15:00:15 [-0500], Clark Williams wrote:
> Found two separate spots where i915 was throwing "sleeping
> function called from invalid context" when running on a
> PREEMPT_RT kernel. In both cases it was from calling
> local_irq_disable prior to taking a spin_lock. Since spin
> locks are converted to rt_mutex_t on PREEMPT_RT this means
> that we might sleep with interrupts disabled.
> 
> Since in both cases the calls were in threaded context on RT
> (irq or ksoftirqd) and in no danger of reentrance, change the
> code to only disable interrupts on non-PREEMPT_RT kernels.
> 
> Signed-off-by: Clark Williams <williams@redhat.com>
> ---
>  drivers/gpu/drm/i915/gt/intel_breadcrumbs.c          | 6 ++++--
>  drivers/gpu/drm/i915/gt/intel_execlists_submission.c | 6 ++++--
>  2 files changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> index 38cc42783dfb..b8bf8d6d3c61 100644
> --- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> +++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> @@ -318,9 +318,11 @@ void __intel_breadcrumbs_park(struct intel_breadcrumbs *b)
>  	/* Kick the work once more to drain the signalers, and disarm the irq */
>  	irq_work_sync(&b->irq_work);
>  	while (READ_ONCE(b->irq_armed) && !atomic_read(&b->active)) {
> -		local_irq_disable();
> +		if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> +			local_irq_disable();
>  		signal_irq_work(&b->irq_work);
> -		local_irq_enable();
> +		if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> +			local_irq_enable();

wouldn't it work to use irq_work_queue() + sync() instead of invoking
the target callback itself? Given that this context is IRQ-enabled then
it should (at least on x86) trigger right away.

>  		cond_resched();
>  	}
>  }
> diff --git a/drivers/gpu/drm/i915/gt/intel_execlists_submission.c b/drivers/gpu/drm/i915/gt/intel_execlists_submission.c
> index fc77592d88a9..0e918831b69f 100644
> --- a/drivers/gpu/drm/i915/gt/intel_execlists_submission.c
> +++ b/drivers/gpu/drm/i915/gt/intel_execlists_submission.c
> @@ -1580,9 +1580,11 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>  
>  static void execlists_dequeue_irq(struct intel_engine_cs *engine)
>  {
> -	local_irq_disable(); /* Suspend interrupts across request submission */
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> +		local_irq_disable(); /* Suspend interrupts across request submission */
>  	execlists_dequeue(engine);

I've been staring at this for a while. Wouldn't it work in invoke
execlists_dequeue() and let execlists_dequeue() do 
	spin_lock_irq(&engine->active.lock);

? This is the only invocation of the function. I don't know what the
expected synchronisation behaviour is. The only thing that could break
is the tail part of the function after the &engine->active.lock has been
dropped.

> -	local_irq_enable(); /* flush irq_work (e.g. breadcrumb enabling) */
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> +		local_irq_enable(); /* flush irq_work (e.g. breadcrumb enabling) */
>  }
>  
>  static void clear_ports(struct i915_request **ports, int count)

Sebastian