dma-buf: add dma_resv_for_each_fence_unlocked v8

Abstract the complexity of iterating over all the fences
in a dma_resv object.

The new loop handles the whole RCU and retry dance and
returns only fences where we can be sure we grabbed the
right one.

v2: fix accessing the shared fences while they might be freed,
    improve kerneldoc, rename _cursor to _iter, add
    dma_resv_iter_is_exclusive, add dma_resv_iter_begin/end

v3: restructor the code, move rcu_read_lock()/unlock() into the
    iterator, add dma_resv_iter_is_restarted()

v4: fix NULL deref when no explicit fence exists, drop superflous
    rcu_read_lock()/unlock() calls.

v5: fix typos in the documentation

v6: fix coding error when excl fence is NULL

v7: one more logic fix

v8: fix index check in dma_resv_iter_is_exclusive()

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> (v7)
Link: https://patchwork.freedesktop.org/patch/msgid/20211005113742.1101-2-christian.koenig@amd.com

diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 84fbe60..3cbcf66 100644 (file)
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -324,6 +324,106 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
 EXPORT_SYMBOL(dma_resv_add_excl_fence);
 
 /**
+ * dma_resv_iter_restart_unlocked - restart the unlocked iterator
+ * @cursor: The dma_resv_iter object to restart
+ *
+ * Restart the unlocked iteration by initializing the cursor object.
+ */
+static void dma_resv_iter_restart_unlocked(struct dma_resv_iter *cursor)
+{
+       cursor->seq = read_seqcount_begin(&cursor->obj->seq);
+       cursor->index = -1;
+       if (cursor->all_fences)
+               cursor->fences = dma_resv_shared_list(cursor->obj);
+       else
+               cursor->fences = NULL;
+       cursor->is_restarted = true;
+}
+
+/**
+ * dma_resv_iter_walk_unlocked - walk over fences in a dma_resv obj
+ * @cursor: cursor to record the current position
+ *
+ * Return all the fences in the dma_resv object which are not yet signaled.
+ * The returned fence has an extra local reference so will stay alive.
+ * If a concurrent modify is detected the whole iteration is started over again.
+ */
+static void dma_resv_iter_walk_unlocked(struct dma_resv_iter *cursor)
+{
+       struct dma_resv *obj = cursor->obj;
+
+       do {
+               /* Drop the reference from the previous round */
+               dma_fence_put(cursor->fence);
+
+               if (cursor->index == -1) {
+                       cursor->fence = dma_resv_excl_fence(obj);
+                       cursor->index++;
+                       if (!cursor->fence)
+                               continue;
+
+               } else if (!cursor->fences ||
+                          cursor->index >= cursor->fences->shared_count) {
+                       cursor->fence = NULL;
+                       break;
+
+               } else {
+                       struct dma_resv_list *fences = cursor->fences;
+                       unsigned int idx = cursor->index++;
+
+                       cursor->fence = rcu_dereference(fences->shared[idx]);
+               }
+               cursor->fence = dma_fence_get_rcu(cursor->fence);
+               if (!cursor->fence || !dma_fence_is_signaled(cursor->fence))
+                       break;
+       } while (true);
+}
+
+/**
+ * dma_resv_iter_first_unlocked - first fence in an unlocked dma_resv obj.
+ * @cursor: the cursor with the current position
+ *
+ * Returns the first fence from an unlocked dma_resv obj.
+ */
+struct dma_fence *dma_resv_iter_first_unlocked(struct dma_resv_iter *cursor)
+{
+       rcu_read_lock();
+       do {
+               dma_resv_iter_restart_unlocked(cursor);
+               dma_resv_iter_walk_unlocked(cursor);
+       } while (read_seqcount_retry(&cursor->obj->seq, cursor->seq));
+       rcu_read_unlock();
+
+       return cursor->fence;
+}
+EXPORT_SYMBOL(dma_resv_iter_first_unlocked);
+
+/**
+ * dma_resv_iter_next_unlocked - next fence in an unlocked dma_resv obj.
+ * @cursor: the cursor with the current position
+ *
+ * Returns the next fence from an unlocked dma_resv obj.
+ */
+struct dma_fence *dma_resv_iter_next_unlocked(struct dma_resv_iter *cursor)
+{
+       bool restart;
+
+       rcu_read_lock();
+       cursor->is_restarted = false;
+       restart = read_seqcount_retry(&cursor->obj->seq, cursor->seq);
+       do {
+               if (restart)
+                       dma_resv_iter_restart_unlocked(cursor);
+               dma_resv_iter_walk_unlocked(cursor);
+               restart = true;
+       } while (read_seqcount_retry(&cursor->obj->seq, cursor->seq));
+       rcu_read_unlock();
+
+       return cursor->fence;
+}
+EXPORT_SYMBOL(dma_resv_iter_next_unlocked);
+
+/**
  * dma_resv_copy_fences - Copy all fences from src to dst.
  * @dst: the destination reservation object
  * @src: the source reservation object

diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index 9100dd3..764138a 100644 (file)
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h
@@ -149,6 +149,101 @@ struct dma_resv {
        struct dma_resv_list __rcu *fence;
 };
 
+/**
+ * struct dma_resv_iter - current position into the dma_resv fences
+ *
+ * Don't touch this directly in the driver, use the accessor function instead.
+ */
+struct dma_resv_iter {
+       /** @obj: The dma_resv object we iterate over */
+       struct dma_resv *obj;
+
+       /** @all_fences: If all fences should be returned */
+       bool all_fences;
+
+       /** @fence: the currently handled fence */
+       struct dma_fence *fence;
+
+       /** @seq: sequence number to check for modifications */
+       unsigned int seq;
+
+       /** @index: index into the shared fences */
+       unsigned int index;
+
+       /** @fences: the shared fences */
+       struct dma_resv_list *fences;
+
+       /** @is_restarted: true if this is the first returned fence */
+       bool is_restarted;
+};
+
+struct dma_fence *dma_resv_iter_first_unlocked(struct dma_resv_iter *cursor);
+struct dma_fence *dma_resv_iter_next_unlocked(struct dma_resv_iter *cursor);
+
+/**
+ * dma_resv_iter_begin - initialize a dma_resv_iter object
+ * @cursor: The dma_resv_iter object to initialize
+ * @obj: The dma_resv object which we want to iterate over
+ * @all_fences: If all fences should be returned or just the exclusive one
+ */
+static inline void dma_resv_iter_begin(struct dma_resv_iter *cursor,
+                                      struct dma_resv *obj,
+                                      bool all_fences)
+{
+       cursor->obj = obj;
+       cursor->all_fences = all_fences;
+       cursor->fence = NULL;
+}
+
+/**
+ * dma_resv_iter_end - cleanup a dma_resv_iter object
+ * @cursor: the dma_resv_iter object which should be cleaned up
+ *
+ * Make sure that the reference to the fence in the cursor is properly
+ * dropped.
+ */
+static inline void dma_resv_iter_end(struct dma_resv_iter *cursor)
+{
+       dma_fence_put(cursor->fence);
+}
+
+/**
+ * dma_resv_iter_is_exclusive - test if the current fence is the exclusive one
+ * @cursor: the cursor of the current position
+ *
+ * Returns true if the currently returned fence is the exclusive one.
+ */
+static inline bool dma_resv_iter_is_exclusive(struct dma_resv_iter *cursor)
+{
+       return cursor->index == 0;
+}
+
+/**
+ * dma_resv_iter_is_restarted - test if this is the first fence after a restart
+ * @cursor: the cursor with the current position
+ *
+ * Return true if this is the first fence in an iteration after a restart.
+ */
+static inline bool dma_resv_iter_is_restarted(struct dma_resv_iter *cursor)
+{
+       return cursor->is_restarted;
+}
+
+/**
+ * dma_resv_for_each_fence_unlocked - unlocked fence iterator
+ * @cursor: a struct dma_resv_iter pointer
+ * @fence: the current fence
+ *
+ * Iterate over the fences in a struct dma_resv object without holding the
+ * &dma_resv.lock and using RCU instead. The cursor needs to be initialized
+ * with dma_resv_iter_begin() and cleaned up with dma_resv_iter_end(). Inside
+ * the iterator a reference to the dma_fence is held and the RCU lock dropped.
+ * When the dma_resv is modified the iteration starts over again.
+ */
+#define dma_resv_for_each_fence_unlocked(cursor, fence)                        \
+       for (fence = dma_resv_iter_first_unlocked(cursor);              \
+            fence; fence = dma_resv_iter_next_unlocked(cursor))
+
 #define dma_resv_held(obj) lockdep_is_held(&(obj)->lock.base)
 #define dma_resv_assert_held(obj) lockdep_assert_held(&(obj)->lock.base)