Add btrfs_end_transaction_throttle to force writers to wait for pending commits

The existing throttle mechanism was often not sufficient to prevent
new writers from coming in and making a given transaction run forever.
This adds an explicit wait at the end of most operations so they will
allow the current transaction to close.

There is no wait inside file_write, inode updates, or cow filling, all which
have different deadlock possibilities.

This is a temporary measure until better asynchronous commit support is
added.  This code leads to stalls as it waits for data=ordered
writeback, and it really needs to be fixed.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 45 insertions, 21 deletions

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 5a1ee0665ae8..69ed5f85a387 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -130,8 +130,27 @@ struct btrfs_trans_handle *btrfs_start_transaction(struct btrfs_root *root,
         return h;
 }
 
-int btrfs_end_transaction(struct btrfs_trans_handle *trans,
-                          struct btrfs_root *root)
+static noinline int wait_for_commit(struct btrfs_root *root,
+                                    struct btrfs_transaction *commit)
+{
+        DEFINE_WAIT(wait);
+        mutex_lock(&root->fs_info->trans_mutex);
+        while(!commit->commit_done) {
+                prepare_to_wait(&commit->commit_wait, &wait,
+                                TASK_UNINTERRUPTIBLE);
+                if (commit->commit_done)
+                        break;
+                mutex_unlock(&root->fs_info->trans_mutex);
+                schedule();
+                mutex_lock(&root->fs_info->trans_mutex);
+        }
+        mutex_unlock(&root->fs_info->trans_mutex);
+        finish_wait(&commit->commit_wait, &wait);
+        return 0;
+}
+
+static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
+                          struct btrfs_root *root, int throttle)
 {
         struct btrfs_transaction *cur_trans;
 
@@ -140,8 +159,18 @@ int btrfs_end_transaction(struct btrfs_trans_handle *trans,
         WARN_ON(cur_trans != trans->transaction);
         WARN_ON(cur_trans->num_writers < 1);
         cur_trans->num_writers--;
+
         if (waitqueue_active(&cur_trans->writer_wait))
                 wake_up(&cur_trans->writer_wait);
+
+        if (cur_trans->in_commit && throttle) {
+                int ret;
+                mutex_unlock(&root->fs_info->trans_mutex);
+                ret = wait_for_commit(root, cur_trans);
+                BUG_ON(ret);
+                mutex_lock(&root->fs_info->trans_mutex);
+        }
+
         put_transaction(cur_trans);
         mutex_unlock(&root->fs_info->trans_mutex);
         memset(trans, 0, sizeof(*trans));
@@ -149,6 +178,18 @@ int btrfs_end_transaction(struct btrfs_trans_handle *trans,
         return 0;
 }
 
+int btrfs_end_transaction(struct btrfs_trans_handle *trans,
+                          struct btrfs_root *root)
+{
+        return __btrfs_end_transaction(trans, root, 0);
+}
+
+int btrfs_end_transaction_throttle(struct btrfs_trans_handle *trans,
+                                   struct btrfs_root *root)
+{
+        return __btrfs_end_transaction(trans, root, 1);
+}
+
 
 int btrfs_write_and_wait_transaction(struct btrfs_trans_handle *trans,
                                      struct btrfs_root *root)
@@ -240,25 +281,6 @@ int btrfs_commit_tree_roots(struct btrfs_trans_handle *trans,
         return 0;
 }
 
-static noinline int wait_for_commit(struct btrfs_root *root,
-                                    struct btrfs_transaction *commit)
-{
-        DEFINE_WAIT(wait);
-        mutex_lock(&root->fs_info->trans_mutex);
-        while(!commit->commit_done) {
-                prepare_to_wait(&commit->commit_wait, &wait,
-                                TASK_UNINTERRUPTIBLE);
-                if (commit->commit_done)
-                        break;
-                mutex_unlock(&root->fs_info->trans_mutex);
-                schedule();
-                mutex_lock(&root->fs_info->trans_mutex);
-        }
-        mutex_unlock(&root->fs_info->trans_mutex);
-        finish_wait(&commit->commit_wait, &wait);
-        return 0;
-}
-
 struct dirty_root {
         struct list_head list;
         struct btrfs_root *root;
@@ -680,6 +702,7 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
                              root->fs_info->btree_inode->i_mapping, GFP_NOFS);
 
         trans->transaction->in_commit = 1;
+printk("trans %Lu in commit\n", trans->transid);
         cur_trans = trans->transaction;
         if (cur_trans->list.prev != &root->fs_info->trans_list) {
                 prev_trans = list_entry(cur_trans->list.prev,
@@ -760,6 +783,7 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
         kfree(pinned_copy);
 
         cur_trans->commit_done = 1;
+printk("trans %Lu done in commit\n", cur_trans->transid);
         root->fs_info->last_trans_committed = cur_trans->transid;
         wake_up(&cur_trans->commit_wait);
         put_transaction(cur_trans);