Btrfs: use ACCESS_ONCE to prevent the optimize accesses to ->last_trans_log_full_commit

->last_trans_log_full_commit may be changed by the other tasks without lock,
so we need prevent the compiler from the optimize access just like
	tmp = fs_info->last_trans_log_full_commit
	if (tmp == ...)
		...

	<do something>

	if (tmp == ...)
		...

In fact, we need get the new value of ->last_trans_log_full_commit during
the second access. Fix it by ACCESS_ONCE().

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 7c449c699beda55ea5b620ec29c38c7f7814ed0c..5a4e10b9ac3e9e12ff5b4a0675b1796fdfd4c81f 100644 (file)
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -2375,14 +2375,14 @@ static int wait_log_commit(struct btrfs_trans_handle *trans,
                                &wait, TASK_UNINTERRUPTIBLE);
                mutex_unlock(&root->log_mutex);
 
-               if (root->fs_info->last_trans_log_full_commit !=
+               if (ACCESS_ONCE(root->fs_info->last_trans_log_full_commit) !=
                    trans->transid && root->log_transid < transid + 2 &&
                    atomic_read(&root->log_commit[index]))
                        schedule();
 
                finish_wait(&root->log_commit_wait[index], &wait);
                mutex_lock(&root->log_mutex);
-       } while (root->fs_info->last_trans_log_full_commit !=
+       } while (ACCESS_ONCE(root->fs_info->last_trans_log_full_commit) !=
                 trans->transid && root->log_transid < transid + 2 &&
                 atomic_read(&root->log_commit[index]));
        return 0;
@@ -2392,12 +2392,12 @@ static void wait_for_writer(struct btrfs_trans_handle *trans,
                            struct btrfs_root *root)
 {
        DEFINE_WAIT(wait);
-       while (root->fs_info->last_trans_log_full_commit !=
+       while (ACCESS_ONCE(root->fs_info->last_trans_log_full_commit) !=
               trans->transid && atomic_read(&root->log_writers)) {
                prepare_to_wait(&root->log_writer_wait,
                                &wait, TASK_UNINTERRUPTIBLE);
                mutex_unlock(&root->log_mutex);
-               if (root->fs_info->last_trans_log_full_commit !=
+               if (ACCESS_ONCE(root->fs_info->last_trans_log_full_commit) !=
                    trans->transid && atomic_read(&root->log_writers))
                        schedule();
                mutex_lock(&root->log_mutex);
@@ -2456,7 +2456,8 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
        }
 
        /* bail out if we need to do a full commit */
-       if (root->fs_info->last_trans_log_full_commit == trans->transid) {
+       if (ACCESS_ONCE(root->fs_info->last_trans_log_full_commit) ==
+           trans->transid) {
                ret = -EAGAIN;
                btrfs_free_logged_extents(log, log_transid);
                mutex_unlock(&root->log_mutex);
@@ -2515,7 +2516,8 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
                        mutex_unlock(&log_root_tree->log_mutex);
                        goto out;
                }
-               root->fs_info->last_trans_log_full_commit = trans->transid;
+               ACCESS_ONCE(root->fs_info->last_trans_log_full_commit) =
+                                                               trans->transid;
                btrfs_wait_marked_extents(log, &log->dirty_log_pages, mark);
                btrfs_free_logged_extents(log, log_transid);
                mutex_unlock(&log_root_tree->log_mutex);
@@ -2547,7 +2549,8 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
         * now that we've moved on to the tree of log tree roots,
         * check the full commit flag again
         */
-       if (root->fs_info->last_trans_log_full_commit == trans->transid) {
+       if (ACCESS_ONCE(root->fs_info->last_trans_log_full_commit) ==
+           trans->transid) {
                blk_finish_plug(&plug);
                btrfs_wait_marked_extents(log, &log->dirty_log_pages, mark);
                btrfs_free_logged_extents(log, log_transid);