Add some rather sophisticated lockless garbage collection
for dma_fence_chain objects.
For this keep all initialized dma_fence_chain nodes an a
queue and trigger garbage collection before a new one is
allocated.
Signed-off-by: Christian König <christian.koenig@xxxxxxx>
---
drivers/dma-buf/dma-fence-chain.c | 160 +++++++++++++++++++++++++-----
include/linux/dma-fence-chain.h | 5 +
2 files changed, 142 insertions(+), 23 deletions(-)
diff --git a/drivers/dma-buf/dma-fence-chain.c b/drivers/dma-buf/dma-fence-chain.c
index 1b4cb3e5cec9..c2f0b69eabb7 100644
--- a/drivers/dma-buf/dma-fence-chain.c
+++ b/drivers/dma-buf/dma-fence-chain.c
@@ -9,8 +9,53 @@
#include <linux/dma-fence-chain.h>
+static struct dma_fence_chain __rcu *fifo_front;
+static struct dma_fence_chain __rcu **fifo_back = &fifo_front;
+
static bool dma_fence_chain_enable_signaling(struct dma_fence *fence);
+/**
+ * dma_fence_chain_enqueue - enqeue a chain node for garbage collection
+ * @chain: the chain node to enqueue
+ *
+ * Add the chain node to the end of the gc fifo.
+ */
+static void dma_fence_chain_enqueue(struct dma_fence_chain *chain)
+{
+ struct dma_fence_chain __rcu **tmp;
+
+ RCU_INIT_POINTER(chain->next, NULL);
+ tmp = xchg((struct dma_fence_chain __force ***)&fifo_back,
+ &chain->next);
+
+ /* This is intentionally unordered since we only need the fifo for gc */
+ rcu_assign_pointer(*tmp, chain);
+}
+
+/**
+ * dma_fence_chain_dequeue - deqeueue a chain node for garbage collection
+ *
+ * Remove the first chain node from the gc fifo while making sure that always
+ * keep at least one node on the fifo for lockless fifo implementation.
+ * Returns the dequeued chain node or NULL.
+ */
+static struct dma_fence_chain *dma_fence_chain_dequeue(void)
+{
+ struct dma_fence_chain *chain, *tmp;
+
+ rcu_read_lock();
+ chain = rcu_dereference(fifo_front);
+ /* Never dequeue the last chain node for lockless fifo */
+ if (unlikely(!chain || !rcu_access_pointer(chain->next))) {
+ rcu_read_unlock();
+ return NULL;
+ }
+ tmp = cmpxchg((struct dma_fence_chain __force **)&fifo_front,
+ chain, rcu_access_pointer(chain->next));
+ rcu_read_unlock();
+ return tmp == chain ? chain : NULL;
+}
+
/**
* dma_fence_chain_get_prev - use RCU to get a reference to the previous fence
* @chain: chain node to get the previous node from
@@ -28,6 +73,43 @@ static struct dma_fence *dma_fence_chain_get_prev(struct dma_fence_chain *chain)
return prev;
}
+/**
+ * dma_fence_chain_try_replace - try to replace the prev node
+ * @chain: Chain node we try to replace prev.
+ * @prev: the old prev node
+ *
+ * Try to replace the previous chain node when it or its containing fence is
+ * signaled. Returns true if we tried, false if we need to wait.
+ */
+static bool dma_fence_chain_try_replace(struct dma_fence_chain *chain,
+ struct dma_fence *prev)
+{
+ struct dma_fence *replacement, *tmp;
+ struct dma_fence_chain *prev_chain;
+
+ prev_chain = to_dma_fence_chain(prev);
+ if (prev_chain) {
+ if (!dma_fence_is_signaled(prev_chain->fence))
+ return false;
+
+ replacement = dma_fence_chain_get_prev(prev_chain);
+ } else {
+ if (!dma_fence_is_signaled(prev))
+ return false;
+
+ replacement = NULL;
+ }
+
+ tmp = cmpxchg((struct dma_fence __force **)&chain->prev, prev,
+ replacement);
+ if (tmp == prev)
+ dma_fence_put(tmp);
+ else
+ dma_fence_put(replacement);
+
+ return true;
+}
+
/**
* dma_fence_chain_walk - chain walking function
* @fence: current chain node
@@ -38,8 +120,8 @@ static struct dma_fence *dma_fence_chain_get_prev(struct dma_fence_chain *chain)
*/
struct dma_fence *dma_fence_chain_walk(struct dma_fence *fence)
{
- struct dma_fence_chain *chain, *prev_chain;
- struct dma_fence *prev, *replacement, *tmp;
+ struct dma_fence_chain *chain;
+ struct dma_fence *prev;
chain = to_dma_fence_chain(fence);
if (!chain) {
@@ -48,26 +130,8 @@ struct dma_fence *dma_fence_chain_walk(struct dma_fence *fence)
}
while ((prev = dma_fence_chain_get_prev(chain))) {
-
- prev_chain = to_dma_fence_chain(prev);
- if (prev_chain) {
- if (!dma_fence_is_signaled(prev_chain->fence))
- break;
-
- replacement = dma_fence_chain_get_prev(prev_chain);
- } else {
- if (!dma_fence_is_signaled(prev))
- break;
-
- replacement = NULL;
- }
-
- tmp = cmpxchg((struct dma_fence __force **)&chain->prev,
- prev, replacement);
- if (tmp == prev)
- dma_fence_put(tmp);
- else
- dma_fence_put(replacement);
+ if (!dma_fence_chain_try_replace(chain, prev))
+ break;
dma_fence_put(prev);
}
@@ -205,7 +269,21 @@ static void dma_fence_chain_release(struct dma_fence *fence)
dma_fence_put(prev);
dma_fence_put(chain->fence);
- dma_fence_free(fence);
+ WRITE_ONCE(chain->fence, NULL);
+
+ /*
+ * Don't garbage collect here to avoid recursion and potential stack
+ * overflow.
+ */
+ chain = dma_fence_chain_dequeue();
+ if (!chain)
+ return;
+
+ if (kref_read(&chain->base.refcount) ||
+ READ_ONCE(chain->fence))
+ dma_fence_chain_enqueue(chain);
+ else
+ dma_fence_free(&chain->base);
}
const struct dma_fence_ops dma_fence_chain_ops = {
@@ -218,6 +296,40 @@ const struct dma_fence_ops dma_fence_chain_ops = {
};
EXPORT_SYMBOL(dma_fence_chain_ops);
+/**
+ * dma_fence_chain_garbage_collect - cleanup chain nodes
+ *
+ * Do some garbage collection and try to release chain nodes.
+ */
+void dma_fence_chain_garbage_collect(void)
+{
+ struct dma_fence_chain *chain = dma_fence_chain_dequeue();
+
+ if (!chain)
+ return;
+
+ if (!dma_fence_get_rcu(&chain->base)) {
+ /* Unused, check if it's also clean */
+ if (likely(!READ_ONCE(chain->fence))) {
+ dma_fence_free(&chain->base);
+ return;
+ }
+
+ } else {
+ struct dma_fence *prev;
+
+ /* Used, do some chain walk */
+ prev = dma_fence_chain_get_prev(chain);
+ if (prev) {
+ dma_fence_chain_try_replace(chain, prev);
+ dma_fence_put(prev);
+ }
+ dma_fence_put(&chain->base);
+ }
+ dma_fence_chain_enqueue(chain);
+}
+EXPORT_SYMBOL(dma_fence_chain_garbage_collect);
+
/**
* dma_fence_chain_init - initialize a fence chain
* @chain: the chain node to initialize
@@ -254,5 +366,7 @@ void dma_fence_chain_init(struct dma_fence_chain *chain,
dma_fence_init(&chain->base, &dma_fence_chain_ops,
&chain->lock, context, seqno);
+ dma_fence_chain_enqueue(chain);
}
+
EXPORT_SYMBOL(dma_fence_chain_init);
diff --git a/include/linux/dma-fence-chain.h b/include/linux/dma-fence-chain.h
index 5f45689a6d2e..b412b5396434 100644
--- a/include/linux/dma-fence-chain.h
+++ b/include/linux/dma-fence-chain.h
@@ -19,6 +19,7 @@
* @base: fence base class
* @lock: spinlock for fence handling
* @prev: previous fence of the chain
+ * @next: next chain node for garbage collection
* @prev_seqno: original previous seqno before garbage collection
* @fence: encapsulated fence
* @cb: callback structure for signaling
@@ -27,6 +28,7 @@
struct dma_fence_chain {
struct dma_fence base;
struct dma_fence __rcu *prev;
+ struct dma_fence_chain __rcu *next;
u64 prev_seqno;
struct dma_fence *fence;
union {
@@ -38,6 +40,8 @@ struct dma_fence_chain {
extern const struct dma_fence_ops dma_fence_chain_ops;
+void dma_fence_chain_garbage_collect(void);
+
/**
* to_dma_fence_chain - cast a fence to a dma_fence_chain
* @fence: fence to cast to a dma_fence_array
@@ -61,6 +65,7 @@ to_dma_fence_chain(struct dma_fence *fence)
*/
static inline struct dma_fence_chain *dma_fence_chain_alloc(void)
{
+ dma_fence_chain_garbage_collect();
return kmalloc(sizeof(struct dma_fence_chain), GFP_KERNEL);
};
--
2.25.1
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