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Subject[PATCH 3.16 222/305] Btrfs: fix race between enabling quotas and subvolume creation
3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Filipe Manana <fdmanana@suse.com>

commit 552f0329c75b3e1d7f9bb8c9e421d37403f192cd upstream.

We have a race between enabling quotas end subvolume creation that cause
subvolume creation to fail with -EINVAL, and the following diagram shows
how it happens:

CPU 0 CPU 1

btrfs_ioctl()
btrfs_ioctl_quota_ctl()
btrfs_quota_enable()
mutex_lock(fs_info->qgroup_ioctl_lock)

btrfs_ioctl()
create_subvol()
btrfs_qgroup_inherit()
-> save fs_info->quota_root
into quota_root
-> stores a NULL value
-> tries to lock the mutex
qgroup_ioctl_lock
-> blocks waiting for
the task at CPU0

-> sets BTRFS_FS_QUOTA_ENABLED in fs_info
-> sets quota_root in fs_info->quota_root
(non-NULL value)

mutex_unlock(fs_info->qgroup_ioctl_lock)

-> checks quota enabled
flag is set
-> returns -EINVAL because
fs_info->quota_root was
NULL before it acquired
the mutex
qgroup_ioctl_lock
-> ioctl returns -EINVAL

Returning -EINVAL to user space will be confusing if all the arguments
passed to the subvolume creation ioctl were valid.

Fix it by grabbing the value from fs_info->quota_root after acquiring
the mutex.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/btrfs/qgroup.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -2024,7 +2024,7 @@ int btrfs_qgroup_inherit(struct btrfs_tr
int ret = 0;
int i;
u64 *i_qgroups;
- struct btrfs_root *quota_root = fs_info->quota_root;
+ struct btrfs_root *quota_root;
struct btrfs_qgroup *srcgroup;
struct btrfs_qgroup *dstgroup;
u32 level_size = 0;
@@ -2034,6 +2034,7 @@ int btrfs_qgroup_inherit(struct btrfs_tr
if (!fs_info->quota_enabled)
goto out;

+ quota_root = fs_info->quota_root;
if (!quota_root) {
ret = -EINVAL;
goto out;
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