[PATCH 05/11] Btrfs: protect fs_info->alloc_start

[Miao Xie <miaox@cn.fujitsu.com>]

fs_info->alloc_start was not protected strictly, it might be changed while
we were accessing it. This patch fixes this problem by using two locks to
protect it (fs_info->chunk_mutex and sb->s_umount). On the read side, we
just need get one of these two locks, and on the write side, we must lock
all of them.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
---
 fs/btrfs/ctree.h | 10 ++++++++++
 fs/btrfs/super.c |  4 ++++
 2 files changed, 14 insertions(+)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 3e672916..201be7d 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1295,6 +1295,16 @@ struct btrfs_fs_info {
 	 * so it is also safe.
 	 */
 	u64 max_inline;
+	/*
+	 * Protected by ->chunk_mutex and sb->s_umount.
+	 *
+	 * The reason that we use two lock to protect it is because only
+	 * remount and mount operations can change it and these two operations
+	 * are under sb->s_umount, but the read side (chunk allocation) can not
+	 * acquire sb->s_umount or the deadlock would happen. So we use two
+	 * locks to protect it. On the write side, we must acquire two locks,
+	 * and on the read side, we just need acquire one of them.
+	 */
 	u64 alloc_start;
 	struct btrfs_transaction *running_transaction;
 	wait_queue_head_t transaction_throttle;
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 073756a..6f0524d 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -519,7 +519,9 @@ int btrfs_parse_options(struct btrfs_root *root, char *options)
 		case Opt_alloc_start:
 			num = match_strdup(&args[0]);
 			if (num) {
+				mutex_lock(&info->chunk_mutex);
 				info->alloc_start = memparse(num, NULL);
+				mutex_unlock(&info->chunk_mutex);
 				kfree(num);
 				printk(KERN_INFO
 					"btrfs: allocations start at %llu\n",
@@ -1289,7 +1291,9 @@ restore:
 	fs_info->mount_opt = old_opts;
 	fs_info->compress_type = old_compress_type;
 	fs_info->max_inline = old_max_inline;
+	mutex_lock(&fs_info->chunk_mutex);
 	fs_info->alloc_start = old_alloc_start;
+	mutex_unlock(&fs_info->chunk_mutex);
 	btrfs_resize_thread_pool(fs_info,
 		old_thread_pool_size, fs_info->thread_pool_size);
 	fs_info->metadata_ratio = old_metadata_ratio;
-- 
1.7.11.7