--- zzzz-none-000/linux-3.10.107/fs/btrfs/locking.c	2017-06-27 09:49:32.000000000 +0000
+++ scorpion-7490-727/linux-3.10.107/fs/btrfs/locking.c	2021-02-04 17:41:59.000000000 +0000
@@ -33,14 +33,14 @@
  */
 void btrfs_set_lock_blocking_rw(struct extent_buffer *eb, int rw)
 {
-	if (eb->lock_nested) {
-		read_lock(&eb->lock);
-		if (eb->lock_nested && current->pid == eb->lock_owner) {
-			read_unlock(&eb->lock);
-			return;
-		}
-		read_unlock(&eb->lock);
-	}
+	/*
+	 * no lock is required.  The lock owner may change if
+	 * we have a read lock, but it won't change to or away
+	 * from us.  If we have the write lock, we are the owner
+	 * and it'll never change.
+	 */
+	if (eb->lock_nested && current->pid == eb->lock_owner)
+		return;
 	if (rw == BTRFS_WRITE_LOCK) {
 		if (atomic_read(&eb->blocking_writers) == 0) {
 			WARN_ON(atomic_read(&eb->spinning_writers) != 1);
@@ -65,19 +65,23 @@
  */
 void btrfs_clear_lock_blocking_rw(struct extent_buffer *eb, int rw)
 {
-	if (eb->lock_nested) {
-		read_lock(&eb->lock);
-		if (eb->lock_nested && current->pid == eb->lock_owner) {
-			read_unlock(&eb->lock);
-			return;
-		}
-		read_unlock(&eb->lock);
-	}
+	/*
+	 * no lock is required.  The lock owner may change if
+	 * we have a read lock, but it won't change to or away
+	 * from us.  If we have the write lock, we are the owner
+	 * and it'll never change.
+	 */
+	if (eb->lock_nested && current->pid == eb->lock_owner)
+		return;
+
 	if (rw == BTRFS_WRITE_LOCK_BLOCKING) {
 		BUG_ON(atomic_read(&eb->blocking_writers) != 1);
 		write_lock(&eb->lock);
 		WARN_ON(atomic_read(&eb->spinning_writers));
 		atomic_inc(&eb->spinning_writers);
+		/*
+		 * atomic_dec_and_test implies a barrier for waitqueue_active
+		 */
 		if (atomic_dec_and_test(&eb->blocking_writers) &&
 		    waitqueue_active(&eb->write_lock_wq))
 			wake_up(&eb->write_lock_wq);
@@ -85,6 +89,9 @@
 		BUG_ON(atomic_read(&eb->blocking_readers) == 0);
 		read_lock(&eb->lock);
 		atomic_inc(&eb->spinning_readers);
+		/*
+		 * atomic_dec_and_test implies a barrier for waitqueue_active
+		 */
 		if (atomic_dec_and_test(&eb->blocking_readers) &&
 		    waitqueue_active(&eb->read_lock_wq))
 			wake_up(&eb->read_lock_wq);
@@ -99,6 +106,9 @@
 void btrfs_tree_read_lock(struct extent_buffer *eb)
 {
 again:
+	BUG_ON(!atomic_read(&eb->blocking_writers) &&
+	       current->pid == eb->lock_owner);
+
 	read_lock(&eb->lock);
 	if (atomic_read(&eb->blocking_writers) &&
 	    current->pid == eb->lock_owner) {
@@ -124,10 +134,11 @@
 }
 
 /*
+ * take a spinning read lock.
  * returns 1 if we get the read lock and 0 if we don't
  * this won't wait for blocking writers
  */
-int btrfs_try_tree_read_lock(struct extent_buffer *eb)
+int btrfs_tree_read_lock_atomic(struct extent_buffer *eb)
 {
 	if (atomic_read(&eb->blocking_writers))
 		return 0;
@@ -144,6 +155,27 @@
 
 /*
  * returns 1 if we get the read lock and 0 if we don't
+ * this won't wait for blocking writers
+ */
+int btrfs_try_tree_read_lock(struct extent_buffer *eb)
+{
+	if (atomic_read(&eb->blocking_writers))
+		return 0;
+
+	if (!read_trylock(&eb->lock))
+		return 0;
+
+	if (atomic_read(&eb->blocking_writers)) {
+		read_unlock(&eb->lock);
+		return 0;
+	}
+	atomic_inc(&eb->read_locks);
+	atomic_inc(&eb->spinning_readers);
+	return 1;
+}
+
+/*
+ * returns 1 if we get the read lock and 0 if we don't
  * this won't wait for blocking writers or readers
  */
 int btrfs_try_tree_write_lock(struct extent_buffer *eb)
@@ -151,6 +183,7 @@
 	if (atomic_read(&eb->blocking_writers) ||
 	    atomic_read(&eb->blocking_readers))
 		return 0;
+
 	write_lock(&eb->lock);
 	if (atomic_read(&eb->blocking_writers) ||
 	    atomic_read(&eb->blocking_readers)) {
@@ -168,14 +201,15 @@
  */
 void btrfs_tree_read_unlock(struct extent_buffer *eb)
 {
-	if (eb->lock_nested) {
-		read_lock(&eb->lock);
-		if (eb->lock_nested && current->pid == eb->lock_owner) {
-			eb->lock_nested = 0;
-			read_unlock(&eb->lock);
-			return;
-		}
-		read_unlock(&eb->lock);
+	/*
+	 * if we're nested, we have the write lock.  No new locking
+	 * is needed as long as we are the lock owner.
+	 * The write unlock will do a barrier for us, and the lock_nested
+	 * field only matters to the lock owner.
+	 */
+	if (eb->lock_nested && current->pid == eb->lock_owner) {
+		eb->lock_nested = 0;
+		return;
 	}
 	btrfs_assert_tree_read_locked(eb);
 	WARN_ON(atomic_read(&eb->spinning_readers) == 0);
@@ -189,17 +223,21 @@
  */
 void btrfs_tree_read_unlock_blocking(struct extent_buffer *eb)
 {
-	if (eb->lock_nested) {
-		read_lock(&eb->lock);
-		if (eb->lock_nested && current->pid == eb->lock_owner) {
-			eb->lock_nested = 0;
-			read_unlock(&eb->lock);
-			return;
-		}
-		read_unlock(&eb->lock);
+	/*
+	 * if we're nested, we have the write lock.  No new locking
+	 * is needed as long as we are the lock owner.
+	 * The write unlock will do a barrier for us, and the lock_nested
+	 * field only matters to the lock owner.
+	 */
+	if (eb->lock_nested && current->pid == eb->lock_owner) {
+		eb->lock_nested = 0;
+		return;
 	}
 	btrfs_assert_tree_read_locked(eb);
 	WARN_ON(atomic_read(&eb->blocking_readers) == 0);
+	/*
+	 * atomic_dec_and_test implies a barrier for waitqueue_active
+	 */
 	if (atomic_dec_and_test(&eb->blocking_readers) &&
 	    waitqueue_active(&eb->read_lock_wq))
 		wake_up(&eb->read_lock_wq);
@@ -212,6 +250,7 @@
  */
 void btrfs_tree_lock(struct extent_buffer *eb)
 {
+	WARN_ON(eb->lock_owner == current->pid);
 again:
 	wait_event(eb->read_lock_wq, atomic_read(&eb->blocking_readers) == 0);
 	wait_event(eb->write_lock_wq, atomic_read(&eb->blocking_writers) == 0);
@@ -244,11 +283,15 @@
 	BUG_ON(blockers > 1);
 
 	btrfs_assert_tree_locked(eb);
+	eb->lock_owner = 0;
 	atomic_dec(&eb->write_locks);
 
 	if (blockers) {
 		WARN_ON(atomic_read(&eb->spinning_writers));
 		atomic_dec(&eb->blocking_writers);
+		/*
+		 * Make sure counter is updated before we wake up waiters.
+		 */
 		smp_mb();
 		if (waitqueue_active(&eb->write_lock_wq))
 			wake_up(&eb->write_lock_wq);