From a83342aa0c8f0ca90057d3837ae8d198186e5153 Mon Sep 17 00:00:00 2001
From: David Sterba <dsterba@suse.com>
Date: Mon, 16 Feb 2015 19:36:47 +0100
Subject: btrfs: add comments to barriers before waitqueue_active

Reduce number of undocumented barriers out there.

Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index 57ee8ca..3a9317c 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -839,6 +839,9 @@ static void free_workspace(int type, struct list_head *workspace)
 	btrfs_compress_op[idx]->free_workspace(workspace);
 	atomic_dec(alloc_workspace);
 wake:
+	/*
+	 * Make sure counter is updated before we wake up waiters.
+	 */
 	smp_mb();
 	if (waitqueue_active(workspace_wait))
 		wake_up(workspace_wait);
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 9f96042..59eb92f 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -10363,8 +10363,7 @@ void btrfs_end_write_no_snapshoting(struct btrfs_root *root)
 {
 	percpu_counter_dec(&root->subv_writers->counter);
 	/*
-	 * Make sure counter is updated before we wake up
-	 * waiters.
+	 * Make sure counter is updated before we wake up waiters.
 	 */
 	smp_mb();
 	if (waitqueue_active(&root->subv_writers->wait))
diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index d7e6baf..03f8630 100644
--- a/fs/btrfs/locking.c
+++ b/fs/btrfs/locking.c
@@ -280,6 +280,9 @@ void btrfs_tree_unlock(struct extent_buffer *eb)
 	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);
diff --git a/fs/btrfs/ordered-data.c b/fs/btrfs/ordered-data.c
index 52170cf..071005f 100644
--- a/fs/btrfs/ordered-data.c
+++ b/fs/btrfs/ordered-data.c
@@ -345,6 +345,9 @@ int btrfs_dec_test_first_ordered_pending(struct inode *inode,
 
 	if (entry->bytes_left == 0) {
 		ret = test_and_set_bit(BTRFS_ORDERED_IO_DONE, &entry->flags);
+		/*
+		 * Implicit memory barrier after test_and_set_bit
+		 */
 		if (waitqueue_active(&entry->wait))
 			wake_up(&entry->wait);
 	} else {
@@ -409,6 +412,9 @@ have_entry:
 
 	if (entry->bytes_left == 0) {
 		ret = test_and_set_bit(BTRFS_ORDERED_IO_DONE, &entry->flags);
+		/*
+		 * Implicit memory barrier after test_and_set_bit
+		 */
 		if (waitqueue_active(&entry->wait))
 			wake_up(&entry->wait);
 	} else {
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index e8e5b5a..3fd70f7 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -861,6 +861,9 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
 	atomic_dec(&cur_trans->num_writers);
 	extwriter_counter_dec(cur_trans, trans->type);
 
+	/*
+	 * Make sure counter is updated before we wake up waiters.
+	 */
 	smp_mb();
 	if (waitqueue_active(&cur_trans->writer_wait))
 		wake_up(&cur_trans->writer_wait);
-- 
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