Re: [PATCH 01/28] dma-buf: add dma_resv_for_each_fence_unlocked v7

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Am 04.10.21 um 11:29 schrieb Tvrtko Ursulin:

On 01/10/2021 11:05, Christian König wrote:
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

Signed-off-by: Christian König <christian.koenig@xxxxxxx>
---
  drivers/dma-buf/dma-resv.c | 100 +++++++++++++++++++++++++++++++++++++
  include/linux/dma-resv.h   |  95 +++++++++++++++++++++++++++++++++++
  2 files changed, 195 insertions(+)

diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 84fbe60629e3..3cbcf66a137e 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -323,6 +323,106 @@ 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);

Worth having an assert dma_fence_get_rcu does not fail here? Not sure that I have seen debug build only asserts though on the DRM core side.

That won't work. It's perfectly valid for dma_fence_get_rcu() to return NULL when we are racing here. Keep in mind that we don't hold any locks.

What we could do is to return NULL and repeat with a new sequence immediately though.


On the bike shedding front, would it be clearer if the continue condition on signaled fences was standalone, using the continue statement? I'd also possibly re-arrange the three if-else blocks so that the end of iteration is not sandwiched between blocks handling exclusive and shared, and flow tweaked a bit, like:

  struct dma_fence *fence = cursor->fence;
  int index = cursor->index;

  dma_fence_put(fence);
  fence = NULL;

next:
  if (index == -1) {
    /* Try picking the exclusive fence. */
    index++;
    fence = dma_resv_excl_fence(obj);
    if (!fence)
        goto next;
  } else if (cursor->fences && index < cursor->fences->shared_count) {
      /* Try picking next shared fence. */
    struct dma_resv_list *fences = cursor->fences;

    fence = rcu_dereference(fences->shared[index++]);
  }

  if (fence) {
      if (dma_fence_is_signaled(fence))
        goto next; /* Skip signaled. */

    fence = dma_fence_get_rcu(fence);
    WARN_ON(!fence);
}

  cursor->fence = fence;
  cursor->index = index;

(I started with a loop here but ended with goto based flow since it ended up more succinct.)

At least if I don't have a handling flaw in there it looks like easier to follow flow to me. Plus picking a not signaled fence works without a reference FWIW.

I strongly don't think that this will work correctly. You need to grab a reference first when you want to call dma_fence_is_signaled(), that's why I used the testbit approach initially.

How does it look to you?

Mhm, let me try to reorder the loop once more.

Thanks,
Christian.


Regards,

Tvrtko

+        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
diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index 9100dd3dc21f..5d7d28cb9008 100644
--- 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 == -1;
+}
+
+/**
+ * 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)





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