From: Jeff Mahoney <jeffm@suse.com>
Subject: btrfs: use spinlock to protect ->caching_block_groups list
Patch-mainline: No, needs discussion about whether we need the commit_root_sem here
References: bsc#1083684

We currently take the commit_root_sem in write mode to access the
fs_info->caching_block_groups list.  The commit that added it is very
old and, from what I can tell, we still just use the commit_root_sem
in write mode here because it's a write lock - not because it's
the commit_root_sem.

This patch adds a new spinlock to protect the list and converts
the write locks to read locks.  We'll revisit whether the lock is needed
post-release.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
---
 fs/btrfs/block-group.c |    6 ++++++
 fs/btrfs/ctree.h       |    1 +
 fs/btrfs/disk-io.c     |    1 +
 fs/btrfs/extent-tree.c |    6 +++++-
 4 files changed, 13 insertions(+), 1 deletion(-)

--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -812,8 +812,10 @@ int btrfs_cache_block_group(struct btrfs
 	}
 
 	down_write(&fs_info->commit_root_sem);
+	spin_lock(&fs_info->caching_block_groups_lock);
 	refcount_inc(&caching_ctl->count);
 	list_add_tail(&caching_ctl->list, &fs_info->caching_block_groups);
+	spin_unlock(&fs_info->caching_block_groups_lock);
 	up_write(&fs_info->commit_root_sem);
 
 	btrfs_get_block_group(cache);
@@ -992,6 +994,7 @@ int btrfs_remove_block_group(struct btrf
 		btrfs_wait_block_group_cache_done(block_group);
 	if (block_group->has_caching_ctl) {
 		down_write(&fs_info->commit_root_sem);
+		spin_lock(&fs_info->caching_block_groups_lock);
 		if (!caching_ctl) {
 			struct btrfs_caching_control *ctl;
 
@@ -1005,6 +1008,7 @@ int btrfs_remove_block_group(struct btrf
 		}
 		if (caching_ctl)
 			list_del_init(&caching_ctl->list);
+		spin_unlock(&fs_info->caching_block_groups_lock);
 		up_write(&fs_info->commit_root_sem);
 		if (caching_ctl) {
 			/* Once for the caching bgs list and once for us. */
@@ -3124,12 +3128,14 @@ int btrfs_free_block_groups(struct btrfs
 	struct rb_node *n;
 
 	down_write(&info->commit_root_sem);
+	spin_lock(&info->caching_block_groups_lock);
 	while (!list_empty(&info->caching_block_groups)) {
 		caching_ctl = list_entry(info->caching_block_groups.next,
 					 struct btrfs_caching_control, list);
 		list_del(&caching_ctl->list);
 		btrfs_put_caching_control(caching_ctl);
 	}
+	spin_unlock(&info->caching_block_groups_lock);
 	up_write(&info->commit_root_sem);
 
 	spin_lock(&info->unused_bgs_lock);
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -677,6 +677,7 @@ struct btrfs_fs_info {
 
 	struct list_head trans_list;
 	struct list_head dead_roots;
+	spinlock_t caching_block_groups_lock;
 	struct list_head caching_block_groups;
 
 	spinlock_t delayed_iput_lock;
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2530,6 +2530,7 @@ int open_ctree(struct super_block *sb,
 	spin_lock_init(&fs_info->qgroup_op_lock);
 	spin_lock_init(&fs_info->buffer_lock);
 	spin_lock_init(&fs_info->unused_bgs_lock);
+	spin_lock_init(&fs_info->caching_block_groups_lock);
 	rwlock_init(&fs_info->tree_mod_log_lock);
 	mutex_init(&fs_info->unused_bg_unpin_mutex);
 	mutex_init(&fs_info->delete_unused_bgs_mutex);
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -2961,7 +2961,8 @@ void btrfs_prepare_extent_commit(struct
 	struct btrfs_caching_control *caching_ctl;
 	struct btrfs_block_group_cache *cache;
 
-	down_write(&fs_info->commit_root_sem);
+	down_read(&fs_info->commit_root_sem);
+	spin_lock(&fs_info->caching_block_groups_lock);
 
 	list_for_each_entry_safe(caching_ctl, next,
 				 &fs_info->caching_block_groups, list) {
@@ -2974,7 +2975,10 @@ void btrfs_prepare_extent_commit(struct
 			cache->last_byte_to_unpin = caching_ctl->progress;
 		}
 	}
+	spin_unlock(&fs_info->caching_block_groups_lock);
+	up_read(&fs_info->commit_root_sem);
 
+	down_write(&fs_info->commit_root_sem);
 	if (fs_info->pinned_extents == &fs_info->freed_extents[0])
 		fs_info->pinned_extents = &fs_info->freed_extents[1];
 	else