Re: [PATCH] nouveau: offload fence uevents work to workqueue

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On 1/29/24 02:50, Dave Airlie wrote:
From: Dave Airlie <airlied@xxxxxxxxxx>

This should break the deadlock between the fctx lock and the irq lock.

This offloads the processing off the work from the irq into a workqueue.

Signed-off-by: Dave Airlie <airlied@xxxxxxxxxx>

Nouveau's scheduler uses a dedicated wq, hence from this perspective it's
safe deferring fence signalling to the kernel global wq. However, I wonder
if we could create deadlocks by building dependency chains into other
drivers / kernel code that, by chance, makes use of the kernel global wq as
well.

Admittedly, even if, it's gonna be extremely unlikely given that
WQ_MAX_ACTIVE == 512. But maybe it'd be safer to use a dedicated wq.

Also, do we need to CC stable?

---
  drivers/gpu/drm/nouveau/nouveau_fence.c | 24 ++++++++++++++++++------
  drivers/gpu/drm/nouveau/nouveau_fence.h |  1 +
  2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c b/drivers/gpu/drm/nouveau/nouveau_fence.c
index ca762ea55413..93f08f9479d8 100644
--- a/drivers/gpu/drm/nouveau/nouveau_fence.c
+++ b/drivers/gpu/drm/nouveau/nouveau_fence.c
@@ -103,6 +103,7 @@ nouveau_fence_context_kill(struct nouveau_fence_chan *fctx, int error)
  void
  nouveau_fence_context_del(struct nouveau_fence_chan *fctx)
  {
+	cancel_work_sync(&fctx->uevent_work);
  	nouveau_fence_context_kill(fctx, 0);
  	nvif_event_dtor(&fctx->event);
  	fctx->dead = 1;
@@ -145,12 +146,13 @@ nouveau_fence_update(struct nouveau_channel *chan, struct nouveau_fence_chan *fc
  	return drop;
  }
-static int
-nouveau_fence_wait_uevent_handler(struct nvif_event *event, void *repv, u32 repc)
+static void
+nouveau_fence_uevent_work(struct work_struct *work)
  {
-	struct nouveau_fence_chan *fctx = container_of(event, typeof(*fctx), event);
+	struct nouveau_fence_chan *fctx = container_of(work, struct nouveau_fence_chan,
+						       uevent_work);
  	unsigned long flags;
-	int ret = NVIF_EVENT_KEEP;
+	int drop = 0;
spin_lock_irqsave(&fctx->lock, flags);
  	if (!list_empty(&fctx->pending)) {
@@ -160,11 +162,20 @@ nouveau_fence_wait_uevent_handler(struct nvif_event *event, void *repv, u32 repc
  		fence = list_entry(fctx->pending.next, typeof(*fence), head);
  		chan = rcu_dereference_protected(fence->channel, lockdep_is_held(&fctx->lock));
  		if (nouveau_fence_update(chan, fctx))
-			ret = NVIF_EVENT_DROP;
+			drop = 1;
  	}
+	if (drop)
+		nvif_event_block(&fctx->event);
+
  	spin_unlock_irqrestore(&fctx->lock, flags);
+}
- return ret;
+static int
+nouveau_fence_wait_uevent_handler(struct nvif_event *event, void *repv, u32 repc)
+{
+	struct nouveau_fence_chan *fctx = container_of(event, typeof(*fctx), event);
+	schedule_work(&fctx->uevent_work);
+	return NVIF_EVENT_KEEP;
  }
void
@@ -178,6 +189,7 @@ nouveau_fence_context_new(struct nouveau_channel *chan, struct nouveau_fence_cha
  	} args;
  	int ret;
+ INIT_WORK(&fctx->uevent_work, nouveau_fence_uevent_work);
  	INIT_LIST_HEAD(&fctx->flip);
  	INIT_LIST_HEAD(&fctx->pending);
  	spin_lock_init(&fctx->lock);
diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.h b/drivers/gpu/drm/nouveau/nouveau_fence.h
index 64d33ae7f356..8bc065acfe35 100644
--- a/drivers/gpu/drm/nouveau/nouveau_fence.h
+++ b/drivers/gpu/drm/nouveau/nouveau_fence.h
@@ -44,6 +44,7 @@ struct nouveau_fence_chan {
  	u32 context;
  	char name[32];
+ struct work_struct uevent_work;
  	struct nvif_event event;
  	int notify_ref, dead, killed;
  };




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