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

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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



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