Btrfs: preserve commit_root for async caching
The async block group caching code uses the commit_root pointer
to get a stable version of the extent allocation tree for scanning.
This copy of the tree root isn't going to change and it significantly
reduces the complexity of the scanning code.
During a commit, we have a loop where we update the extent allocation
tree root. We need to loop because updating the root pointer in
the tree of tree roots may allocate blocks which may change the
extent allocation tree.
Right now the commit_root pointer is changed inside this loop. It
is more correct to change the commit_root pointer only after all the
looping is done.
Signed-off-by: Yan Zheng <zheng.yan@oracle.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 17ad92c..38eeb6c 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -827,6 +827,7 @@
struct mutex drop_mutex;
struct mutex volume_mutex;
struct mutex tree_reloc_mutex;
+ struct rw_semaphore extent_commit_sem;
/*
* this protects the ordered operations list only while we are
@@ -961,9 +962,6 @@
/* the node lock is held while changing the node pointer */
spinlock_t node_lock;
- /* taken when updating the commit root */
- struct rw_semaphore commit_root_sem;
-
struct extent_buffer *commit_root;
struct btrfs_root *log_root;
struct btrfs_root *reloc_root;
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 3a9b887..3cf4cfa 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -907,7 +907,6 @@
spin_lock_init(&root->inode_lock);
mutex_init(&root->objectid_mutex);
mutex_init(&root->log_mutex);
- init_rwsem(&root->commit_root_sem);
init_waitqueue_head(&root->log_writer_wait);
init_waitqueue_head(&root->log_commit_wait[0]);
init_waitqueue_head(&root->log_commit_wait[1]);
@@ -1624,6 +1623,7 @@
mutex_init(&fs_info->cleaner_mutex);
mutex_init(&fs_info->volume_mutex);
mutex_init(&fs_info->tree_reloc_mutex);
+ init_rwsem(&fs_info->extent_commit_sem);
btrfs_init_free_cluster(&fs_info->meta_alloc_cluster);
btrfs_init_free_cluster(&fs_info->data_alloc_cluster);
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index fadf69a..2fe21fa 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -267,7 +267,7 @@
last = max_t(u64, block_group->key.objectid, BTRFS_SUPER_INFO_OFFSET);
again:
/* need to make sure the commit_root doesn't disappear */
- down_read(&fs_info->extent_root->commit_root_sem);
+ down_read(&fs_info->extent_commit_sem);
/*
* We don't want to deadlock with somebody trying to allocate a new
@@ -304,7 +304,7 @@
if (need_resched()) {
btrfs_release_path(fs_info->extent_root, path);
- up_read(&fs_info->extent_root->commit_root_sem);
+ up_read(&fs_info->extent_commit_sem);
cond_resched();
goto again;
}
@@ -345,7 +345,7 @@
err:
btrfs_free_path(path);
- up_read(&fs_info->extent_root->commit_root_sem);
+ up_read(&fs_info->extent_commit_sem);
atomic_dec(&block_group->space_info->caching_threads);
wake_up(&block_group->caching_q);
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index e51d2bc..de48e4e 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -42,10 +42,8 @@
static noinline void switch_commit_root(struct btrfs_root *root)
{
- down_write(&root->commit_root_sem);
free_extent_buffer(root->commit_root);
root->commit_root = btrfs_root_node(root);
- up_write(&root->commit_root_sem);
}
/*
@@ -466,7 +464,10 @@
ret = btrfs_write_dirty_block_groups(trans, root);
BUG_ON(ret);
}
- switch_commit_root(root);
+
+ if (root != root->fs_info->extent_root)
+ switch_commit_root(root);
+
return 0;
}
@@ -499,6 +500,11 @@
update_cowonly_root(trans, root);
}
+
+ down_write(&fs_info->extent_commit_sem);
+ switch_commit_root(fs_info->extent_root);
+ up_write(&fs_info->extent_commit_sem);
+
return 0;
}