drm/v3d: create a dedicated lock for dma fence

Don't mix dma fence lock with the active_job lock. Use fence_lock to
protect the dma fence used by drm scheduler when signalling a job
completion and queue_lock to protect concurrent access to active bin job
in OOM and stats collection for a given file priv. The issue was
uncovered when PREEMPT_RT on with a system freeze when opening multiple
Chromium tabs on Raspberry Pi 5.

Link: https://github.com/raspberrypi/linux/issues/7035
Fixes: fa6a20c874 ("drm/v3d: Address race-condition between per-fd GPU stats and fd release")
Signed-off-by: Melissa Wen <mwen@igalia.com>
Acked-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://lore.kernel.org/r/20250916172022.2779837-1-mwen@igalia.com

drivers/gpu/drm/v3d/v3d_drv.h

@@ -62,6 +62,8 @@ struct v3d_queue_state {
 	/* Currently active job for this queue */
 	struct v3d_job *active_job;
 	spinlock_t queue_lock;
+	/* Protect dma fence for signalling job completion */
+	spinlock_t fence_lock;
 };
 
 /* Performance monitor object. The perform lifetime is controlled by userspace

drivers/gpu/drm/v3d/v3d_fence.c

@@ -15,7 +15,7 @@ struct dma_fence *v3d_fence_create(struct v3d_dev *v3d, enum v3d_queue q)
 	fence->dev = &v3d->drm;
 	fence->queue = q;
 	fence->seqno = ++queue->emit_seqno;
-	dma_fence_init(&fence->base, &v3d_fence_ops, &queue->queue_lock,
+	dma_fence_init(&fence->base, &v3d_fence_ops, &queue->fence_lock,
 		       queue->fence_context, fence->seqno);
 
 	return &fence->base;

drivers/gpu/drm/v3d/v3d_gem.c

@@ -274,6 +274,7 @@ v3d_gem_init(struct drm_device *dev)
 		seqcount_init(&queue->stats.lock);
 
 		spin_lock_init(&queue->queue_lock);
+		spin_lock_init(&queue->fence_lock);
 	}
 
 	spin_lock_init(&v3d->mm_lock);