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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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