nouveau: push event block/allowing out of the fence context

There is a deadlock between the irq and fctx locks,
the irq handling takes irq then fctx lock
the fence signalling takes fctx then irq lock

This splits the fence signalling path so the code that hits
the irq lock is done in a separate work queue.

This seems to fix crashes/hangs when using nouveau gsp with
i915 primary GPU.

Signed-off-by: Dave Airlie <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/nouveau/nouveau_fence.c

···
 	if (test_bit(DMA_FENCE_FLAG_USER_BITS, &fence->base.flags)) {
 		struct nouveau_fence_chan *fctx = nouveau_fctx(fence);
 
-		if (!--fctx->notify_ref)
+		if (atomic_dec_and_test(&fctx->notify_ref))
 			drop = 1;
 	}
 
···
 void
 nouveau_fence_context_del(struct nouveau_fence_chan *fctx)
 {
+	cancel_work_sync(&fctx->allow_block_work);
 	nouveau_fence_context_kill(fctx, 0);
 	nvif_event_dtor(&fctx->event);
 	fctx->dead = 1;
···
 	return ret;
 }
 
+static void
+nouveau_fence_work_allow_block(struct work_struct *work)
+{
+	struct nouveau_fence_chan *fctx = container_of(work, struct nouveau_fence_chan,
+						       allow_block_work);
+
+	if (atomic_read(&fctx->notify_ref) == 0)
+		nvif_event_block(&fctx->event);
+	else
+		nvif_event_allow(&fctx->event);
+}
+
 void
 nouveau_fence_context_new(struct nouveau_channel *chan, struct nouveau_fence_chan *fctx)
 {
···
 	} args;
 	int ret;
 
+	INIT_WORK(&fctx->allow_block_work, nouveau_fence_work_allow_block);
 	INIT_LIST_HEAD(&fctx->flip);
 	INIT_LIST_HEAD(&fctx->pending);
 	spin_lock_init(&fctx->lock);
···
 	struct nouveau_fence *fence = from_fence(f);
 	struct nouveau_fence_chan *fctx = nouveau_fctx(fence);
 	bool ret;
+	bool do_work;
 
-	if (!fctx->notify_ref++)
-		nvif_event_allow(&fctx->event);
+	if (atomic_inc_return(&fctx->notify_ref) == 0)
+		do_work = true;
 
 	ret = nouveau_fence_no_signaling(f);
 	if (ret)
 		set_bit(DMA_FENCE_FLAG_USER_BITS, &fence->base.flags);
-	else if (!--fctx->notify_ref)
-		nvif_event_block(&fctx->event);
+	else if (atomic_dec_and_test(&fctx->notify_ref))
+		do_work = true;
+
+	if (do_work)
+		schedule_work(&fctx->allow_block_work);
 
 	return ret;
 }

drivers/gpu/drm/nouveau/nouveau_fence.h

···
 #define __NOUVEAU_FENCE_H__
 
 #include <linux/dma-fence.h>
+#include <linux/workqueue.h>
 #include <nvif/event.h>
 
 struct nouveau_drm;
···
 	char name[32];
 
 	struct nvif_event event;
-	int notify_ref, dead, killed;
+	struct work_struct allow_block_work;
+	atomic_t notify_ref;
+	int dead, killed;
 };
 
 struct nouveau_fence_priv {