Re: [PATCH] drm/msm/dpu: Avoid ABBA deadlock between IRQ modules

[Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx>]

On 10/06/2021 02:15, Bjorn Andersson wrote:
> Handling of the interrupt callback lists is done in dpu_core_irq.c,
> under the "cb_lock" spinlock. When these operations results in the need
> for enableing or disabling the IRQ in the hardware the code jumps to
> dpu_hw_interrupts.c, which protects its operations with "irq_lock"
> spinlock.
>
> When an interrupt fires, dpu_hw_intr_dispatch_irq() inspects the
> hardware state while holding the "irq_lock" spinlock and jumps to
> dpu_core_irq_callback_handler() to invoke the registered handlers, which
> traverses the callback list under the "cb_lock" spinlock.
>
> As such, in the event that these happens concurrently we'll end up with
> a deadlock.
>
> Prior to '1c1e7763a6d4 ("drm/msm/dpu: simplify IRQ enabling/disabling")'
> the enable/disable of the hardware interrupt was done outside the
> "cb_lock" region, optimitically by using an atomic enable-counter for
> each interrupt and an warning print if someone changed the list between
> the atomic_read and the time the operation concluded.
>
> Rather than re-introducing the large array of atomics, serialize the
> register/unregister operations under a single mutex.
>
> Fixes: 1c1e7763a6d4 ("drm/msm/dpu: simplify IRQ enabling/disabling")
> Signed-off-by: Bjorn Andersson <bjorn.andersson@xxxxxxxxxx>

I have been thinking about this for quite some time. I'm still not 
confident about the proposed scheme.

What more intrusive, but more obvious change:
 - Drop dpu_kms->irq_obj.cb_lock alltogether. Use hw_intr's irq_lock 
instead in the register/unregister path and no locks in the callback itself.
 - Do not take locks in the dpu_hw_intr_enable_irq/disable_irq (as we 
are already locked outside).

The core_irq is the only user of the hw_intr framework. In fact I'd like 
to squash them together at some point (if I get some time, I'll send 
patches during this cycle).


> ---
>   drivers/gpu/drm/msm/disp/dpu1/dpu_core_irq.c | 10 +++++++---
>   drivers/gpu/drm/msm/disp/dpu1/dpu_kms.h      |  2 ++
>   2 files changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_core_irq.c b/drivers/gpu/drm/msm/disp/dpu1/dpu_core_irq.c
> index 4f110c428b60..62bbe35eff7b 100644
> --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_core_irq.c
> +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_core_irq.c
> @@ -82,11 +82,13 @@ int dpu_core_irq_register_callback(struct dpu_kms *dpu_kms, int irq_idx,
>   
>   	DPU_DEBUG("[%pS] irq_idx=%d\n", __builtin_return_address(0), irq_idx);
>   
> +	mutex_lock(&dpu_kms->irq_obj.hw_enable_lock);
>   	spin_lock_irqsave(&dpu_kms->irq_obj.cb_lock, irq_flags);
>   	trace_dpu_core_irq_register_callback(irq_idx, register_irq_cb);
>   	list_del_init(&register_irq_cb->list);
>   	list_add_tail(&register_irq_cb->list,
>   			&dpu_kms->irq_obj.irq_cb_tbl[irq_idx]);
> +	spin_unlock_irqrestore(&dpu_kms->irq_obj.cb_lock, irq_flags);
>   	if (list_is_first(&register_irq_cb->list,
>   			&dpu_kms->irq_obj.irq_cb_tbl[irq_idx])) {
>   		int ret = dpu_kms->hw_intr->ops.enable_irq(
> @@ -96,8 +98,7 @@ int dpu_core_irq_register_callback(struct dpu_kms *dpu_kms, int irq_idx,
>   			DPU_ERROR("Fail to enable IRQ for irq_idx:%d\n",
>   					irq_idx);
>   	}
> -
> -	spin_unlock_irqrestore(&dpu_kms->irq_obj.cb_lock, irq_flags);
> +	mutex_unlock(&dpu_kms->irq_obj.hw_enable_lock);
>   
>   	return 0;
>   }
> @@ -127,9 +128,11 @@ int dpu_core_irq_unregister_callback(struct dpu_kms *dpu_kms, int irq_idx,
>   
>   	DPU_DEBUG("[%pS] irq_idx=%d\n", __builtin_return_address(0), irq_idx);
>   
> +	mutex_lock(&dpu_kms->irq_obj.hw_enable_lock);
>   	spin_lock_irqsave(&dpu_kms->irq_obj.cb_lock, irq_flags);
>   	trace_dpu_core_irq_unregister_callback(irq_idx, register_irq_cb);
>   	list_del_init(&register_irq_cb->list);
> +	spin_unlock_irqrestore(&dpu_kms->irq_obj.cb_lock, irq_flags);
>   	/* empty callback list but interrupt is still enabled */
>   	if (list_empty(&dpu_kms->irq_obj.irq_cb_tbl[irq_idx])) {
>   		int ret = dpu_kms->hw_intr->ops.disable_irq(
> @@ -140,7 +143,7 @@ int dpu_core_irq_unregister_callback(struct dpu_kms *dpu_kms, int irq_idx,
>   					irq_idx);
>   		DPU_DEBUG("irq_idx=%d ret=%d\n", irq_idx, ret);
>   	}
> -	spin_unlock_irqrestore(&dpu_kms->irq_obj.cb_lock, irq_flags);
> +	mutex_unlock(&dpu_kms->irq_obj.hw_enable_lock);
>   
>   	return 0;
>   }
> @@ -207,6 +210,7 @@ void dpu_core_irq_preinstall(struct dpu_kms *dpu_kms)
>   	dpu_disable_all_irqs(dpu_kms);
>   	pm_runtime_put_sync(&dpu_kms->pdev->dev);
>   
> +	mutex_init(&dpu_kms->irq_obj.hw_enable_lock);
>   	spin_lock_init(&dpu_kms->irq_obj.cb_lock);
>   
>   	/* Create irq callbacks for all possible irq_idx */
> diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_kms.h b/drivers/gpu/drm/msm/disp/dpu1/dpu_kms.h
> index f6840b1af6e4..5a162caea29d 100644
> --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_kms.h
> +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_kms.h
> @@ -83,6 +83,7 @@ struct dpu_irq_callback {
>    * @total_irq:    total number of irq_idx obtained from HW interrupts mapping
>    * @irq_cb_tbl:   array of IRQ callbacks setting
>    * @cb_lock:      callback lock
> + * @hw_enable_lock: lock to synchronize callback register and unregister
>    * @debugfs_file: debugfs file for irq statistics
>    */
>   struct dpu_irq {
> @@ -90,6 +91,7 @@ struct dpu_irq {
>   	struct list_head *irq_cb_tbl;
>   	atomic_t *irq_counts;
>   	spinlock_t cb_lock;
> +	struct mutex hw_enable_lock;
>   };
>   
>   struct dpu_kms {


--
With best wishes
Dmitry