btrfs: fix races on root_log_ctx lists

btrfs_remove_all_log_ctxs takes a shortcut where it avoids walking the
list because it knows all of the waiters are patiently waiting for the
commit to finish.

But, there's a small race where btrfs_sync_log can remove itself from
the list if it finds a log commit is already done.  Also, it uses
list_del_init() to remove itself from the list, but there's no way to
know if btrfs_remove_all_log_ctxs has already run, so we don't know for
sure if it is safe to call list_del_init().

This gets rid of all the shortcuts for btrfs_remove_all_log_ctxs(), and
just calls it with the proper locking.

This is part two of the corruption fixed by cbd60aa7cd1.  I should have
done this in the first place, but convinced myself the optimizations were
safe.  A 12 hour run of dbench 2048 will eventually trigger a list debug
WARN_ON for the list_del_init() in btrfs_sync_log().

Fixes: d1433debe7f4346cf9fc0dafc71c3137d2a97bc4
Reported-by: Dave Jones <davej@codemonkey.org.uk>
cc: stable@vger.kernel.org # 3.15+
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 6 insertions, 14 deletions

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 688df71c1bf7..4b1c0a6eee04 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -2713,14 +2713,12 @@ static inline void btrfs_remove_all_log_ctxs(struct btrfs_root *root,
                                              int index, int error)
 {
         struct btrfs_log_ctx *ctx;
+        struct btrfs_log_ctx *safe;
 
-        if (!error) {
-                INIT_LIST_HEAD(&root->log_ctxs[index]);
-                return;
-        }
-
-        list_for_each_entry(ctx, &root->log_ctxs[index], list)
+        list_for_each_entry_safe(ctx, safe, &root->log_ctxs[index], list) {
+                list_del_init(&ctx->list);
                 ctx->log_ret = error;
+        }
 
         INIT_LIST_HEAD(&root->log_ctxs[index]);
 }
@@ -2961,13 +2959,9 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
         mutex_unlock(&root->log_mutex);
 
 out_wake_log_root:
-        /*
-         * We needn't get log_mutex here because we are sure all
-         * the other tasks are blocked.
-         */
+        mutex_lock(&log_root_tree->log_mutex);
         btrfs_remove_all_log_ctxs(log_root_tree, index2, ret);
 
-        mutex_lock(&log_root_tree->log_mutex);
         log_root_tree->log_transid_committed++;
         atomic_set(&log_root_tree->log_commit[index2], 0);
         mutex_unlock(&log_root_tree->log_mutex);
@@ -2978,10 +2972,8 @@ out_wake_log_root:
         if (waitqueue_active(&log_root_tree->log_commit_wait[index2]))
                 wake_up(&log_root_tree->log_commit_wait[index2]);
 out:
-        /* See above. */
-        btrfs_remove_all_log_ctxs(root, index1, ret);
-
         mutex_lock(&root->log_mutex);
+        btrfs_remove_all_log_ctxs(root, index1, ret);
         root->log_transid_committed++;
         atomic_set(&root->log_commit[index1], 0);
         mutex_unlock(&root->log_mutex);