dma-buf: inline spinlock for fence protection v5

Implement per-fence spinlocks, allowing implementations to not give an
external spinlock to protect the fence internal state. Instead a spinlock
embedded into the fence structure itself is used in this case.

Shared spinlocks have the problem that implementations need to guarantee
that the lock lives at least as long all fences referencing them.

Using a per-fence spinlock allows completely decoupling spinlock producer
and consumer life times, simplifying the handling in most use cases.

v2: improve naming, coverage and function documentation
v3: fix one additional locking in the selftests
v4: separate out some changes to make the patch smaller,
    fix one amdgpu crash found by CI systems
v5: improve comments

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/r/20260219160822.1529-5-christian.koenig@amd.com

drivers/dma-buf/dma-fence.c

@@ -343,7 +343,6 @@ void __dma_fence_might_wait(void)
 }
 #endif
 
-
 /**
  * dma_fence_signal_timestamp_locked - signal completion of a fence
  * @fence: the fence to signal
@@ -1070,7 +1069,6 @@ static void
 __dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
 	         spinlock_t *lock, u64 context, u64 seqno, unsigned long flags)
 {
-	BUG_ON(!lock);
 	BUG_ON(!ops || !ops->get_driver_name || !ops->get_timeline_name);
 
 	kref_init(&fence->refcount);
@@ -1082,10 +1080,15 @@ __dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
 	 */
 	RCU_INIT_POINTER(fence->ops, ops);
 	INIT_LIST_HEAD(&fence->cb_list);
-	fence->lock = lock;
 	fence->context = context;
 	fence->seqno = seqno;
 	fence->flags = flags | BIT(DMA_FENCE_FLAG_INITIALIZED_BIT);
+	if (lock) {
+		fence->extern_lock = lock;
+	} else {
+		spin_lock_init(&fence->inline_lock);
+		fence->flags |= BIT(DMA_FENCE_FLAG_INLINE_LOCK_BIT);
+	}
 	fence->error = 0;
 
 	trace_dma_fence_init(fence);
@@ -1095,7 +1098,7 @@ __dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
  * dma_fence_init - Initialize a custom fence.
  * @fence: the fence to initialize
  * @ops: the dma_fence_ops for operations on this fence
- * @lock: the irqsafe spinlock to use for locking this fence
+ * @lock: optional irqsafe spinlock to use for locking this fence
  * @context: the execution context this fence is run on
  * @seqno: a linear increasing sequence number for this context
  *
@@ -1105,6 +1108,10 @@ __dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
  *
  * context and seqno are used for easy comparison between fences, allowing
  * to check which fence is later by simply using dma_fence_later().
+ *
+ * It is strongly discouraged to provide an external lock because this couples
+ * lock and fence life time. This is only allowed for legacy use cases when
+ * multiple fences need to be prevented from signaling out of order.
  */
 void
 dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
@@ -1118,7 +1125,7 @@ EXPORT_SYMBOL(dma_fence_init);
  * dma_fence_init64 - Initialize a custom fence with 64-bit seqno support.
  * @fence: the fence to initialize
  * @ops: the dma_fence_ops for operations on this fence
- * @lock: the irqsafe spinlock to use for locking this fence
+ * @lock: optional irqsafe spinlock to use for locking this fence
  * @context: the execution context this fence is run on
  * @seqno: a linear increasing sequence number for this context
  *
@@ -1128,6 +1135,10 @@ EXPORT_SYMBOL(dma_fence_init);
  *
  * Context and seqno are used for easy comparison between fences, allowing
  * to check which fence is later by simply using dma_fence_later().
+ *
+ * It is strongly discouraged to provide an external lock because this couples
+ * lock and fence life time. This is only allowed for legacy use cases when
+ * multiple fences need to be prevented from signaling out of order.
  */
 void
 dma_fence_init64(struct dma_fence *fence, const struct dma_fence_ops *ops,

drivers/dma-buf/sync_debug.h

@@ -47,7 +47,7 @@ struct sync_timeline {
 
 static inline struct sync_timeline *dma_fence_parent(struct dma_fence *fence)
 {
-	return container_of(fence->lock, struct sync_timeline, lock);
+	return container_of(fence->extern_lock, struct sync_timeline, lock);
 }
 
 /**

drivers/gpu/drm/drm_crtc.c

@@ -159,7 +159,7 @@ static const struct dma_fence_ops drm_crtc_fence_ops;
 static struct drm_crtc *fence_to_crtc(struct dma_fence *fence)
 {
 	BUG_ON(rcu_access_pointer(fence->ops) != &drm_crtc_fence_ops);
-	return container_of(fence->lock, struct drm_crtc, fence_lock);
+	return container_of(fence->extern_lock, struct drm_crtc, fence_lock);
 }
 
 static const char *drm_crtc_fence_get_driver_name(struct dma_fence *fence)

drivers/gpu/drm/drm_writeback.c

@@ -81,7 +81,7 @@
  *	From userspace, this property will always read as zero.
  */
 
-#define fence_to_wb_connector(x) container_of(x->lock, \
+#define fence_to_wb_connector(x) container_of(x->extern_lock, \
 					      struct drm_writeback_connector, \
 					      fence_lock)
 

drivers/gpu/drm/nouveau/nouveau_fence.c

@@ -41,7 +41,8 @@ static const struct dma_fence_ops nouveau_fence_ops_legacy;
 static inline struct nouveau_fence_chan *
 nouveau_fctx(struct nouveau_fence *fence)
 {
-	return container_of(fence->base.lock, struct nouveau_fence_chan, lock);
+	return container_of(fence->base.extern_lock, struct nouveau_fence_chan,
+			    lock);
 }
 
 static bool

drivers/gpu/drm/qxl/qxl_release.c

@@ -62,7 +62,8 @@ static long qxl_fence_wait(struct dma_fence *fence, bool intr,
 	struct qxl_device *qdev;
 	unsigned long cur, end = jiffies + timeout;
 
-	qdev = container_of(fence->lock, struct qxl_device, release_lock);
+	qdev = container_of(fence->extern_lock, struct qxl_device,
+			    release_lock);
 
 	if (!wait_event_timeout(qdev->release_event,
 				(dma_fence_is_signaled(fence) ||

drivers/gpu/drm/vmwgfx/vmwgfx_fence.c

@@ -47,7 +47,8 @@ struct vmw_event_fence_action {
 static struct vmw_fence_manager *
 fman_from_fence(struct vmw_fence_obj *fence)
 {
-	return container_of(fence->base.lock, struct vmw_fence_manager, lock);
+	return container_of(fence->base.extern_lock, struct vmw_fence_manager,
+			    lock);
 }
 
 static void vmw_fence_obj_destroy(struct dma_fence *f)

drivers/gpu/drm/xe/xe_hw_fence.c

@@ -124,7 +124,8 @@ static struct xe_hw_fence *to_xe_hw_fence(struct dma_fence *fence);
 
 static struct xe_hw_fence_irq *xe_hw_fence_irq(struct xe_hw_fence *fence)
 {
-	return container_of(fence->dma.lock, struct xe_hw_fence_irq, lock);
+	return container_of(fence->dma.extern_lock, struct xe_hw_fence_irq,
+			    lock);
 }
 
 static const char *xe_hw_fence_get_driver_name(struct dma_fence *dma_fence)

include/linux/dma-fence.h

@@ -34,7 +34,8 @@ struct seq_file;
  * @ops: dma_fence_ops associated with this fence
  * @rcu: used for releasing fence with kfree_rcu
  * @cb_list: list of all callbacks to call
- * @lock: spin_lock_irqsave used for locking
+ * @extern_lock: external spin_lock_irqsave used for locking (deprecated)
+ * @inline_lock: alternative internal spin_lock_irqsave used for locking
  * @context: execution context this fence belongs to, returned by
  *           dma_fence_context_alloc()
  * @seqno: the sequence number of this fence inside the execution context,
@@ -49,6 +50,7 @@ struct seq_file;
  * of the time.
  *
  * DMA_FENCE_FLAG_INITIALIZED_BIT - fence was initialized
+ * DMA_FENCE_FLAG_INLINE_LOCK_BIT - use inline spinlock instead of external one
  * DMA_FENCE_FLAG_SIGNALED_BIT - fence is already signaled
  * DMA_FENCE_FLAG_TIMESTAMP_BIT - timestamp recorded for fence signaling
  * DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT - enable_signaling might have been called
@@ -66,7 +68,10 @@ struct seq_file;
  * been completed, or never called at all.
  */
 struct dma_fence {
-	spinlock_t *lock;
+	union {
+		spinlock_t *extern_lock;
+		spinlock_t inline_lock;
+	};
 	const struct dma_fence_ops __rcu *ops;
 	/*
 	 * We clear the callback list on kref_put so that by the time we
@@ -100,6 +105,7 @@ struct dma_fence {
 
 enum dma_fence_flag_bits {
 	DMA_FENCE_FLAG_INITIALIZED_BIT,
+	DMA_FENCE_FLAG_INLINE_LOCK_BIT,
 	DMA_FENCE_FLAG_SEQNO64_BIT,
 	DMA_FENCE_FLAG_SIGNALED_BIT,
 	DMA_FENCE_FLAG_TIMESTAMP_BIT,
@@ -381,11 +387,12 @@ dma_fence_get_rcu_safe(struct dma_fence __rcu **fencep)
  * dma_fence_spinlock - return pointer to the spinlock protecting the fence
  * @fence: the fence to get the lock from
  *
- * Return the pointer to the extern lock.
+ * Return either the pointer to the embedded or the external spin lock.
  */
 static inline spinlock_t *dma_fence_spinlock(struct dma_fence *fence)
 {
-	return fence->lock;
+	return test_bit(DMA_FENCE_FLAG_INLINE_LOCK_BIT, &fence->flags) ?
+		&fence->inline_lock : fence->extern_lock;
 }
 
 /**
@@ -396,7 +403,7 @@ static inline spinlock_t *dma_fence_spinlock(struct dma_fence *fence)
  * Lock the fence, preventing it from changing to the signaled state.
  */
 #define dma_fence_lock_irqsave(fence, flags)	\
-	spin_lock_irqsave(fence->lock, flags)
+	spin_lock_irqsave(dma_fence_spinlock(fence), flags)
 
 /**
  * dma_fence_unlock_irqrestore - unlock the fence and irqrestore
@@ -406,7 +413,7 @@ static inline spinlock_t *dma_fence_spinlock(struct dma_fence *fence)
  * Unlock the fence, allowing it to change it's state to signaled again.
  */
 #define dma_fence_unlock_irqrestore(fence, flags)	\
-	spin_unlock_irqrestore(fence->lock, flags)
+	spin_unlock_irqrestore(dma_fence_spinlock(fence), flags)
 
 /**
  * dma_fence_assert_held - lockdep assertion that fence is locked