On 2018/11/19 下午5:48, fdman...@kernel.org wrote: > From: Filipe Manana <fdman...@suse.com> > > If the quota enable and snapshot creation ioctls are called concurrently > we can get into a deadlock where the task enabling quotas will deadlock > on the fs_info->qgroup_ioctl_lock mutex because it attempts to lock it > twice. The following time diagram shows how this happens. > > CPU 0 CPU 1 > > btrfs_ioctl() > btrfs_ioctl_quota_ctl() > btrfs_quota_enable() > mutex_lock(fs_info->qgroup_ioctl_lock) > btrfs_start_transaction() > > btrfs_ioctl() > btrfs_ioctl_snap_create_v2 > create_snapshot() > --> adds snapshot to the > list pending_snapshots > of the current > transaction > > btrfs_commit_transaction() > create_pending_snapshots() > create_pending_snapshot() > qgroup_account_snapshot() > btrfs_qgroup_inherit() > mutex_lock(fs_info->qgroup_ioctl_lock) > --> deadlock, mutex already locked > by this task at > btrfs_quota_enable()
The backtrace looks valid.
>
> So fix this by adding a flag to the transaction handle that signals if the
> transaction is being used for enabling quotas (only seen by the task doing
> it) and do not lock the mutex qgroup_ioctl_lock at btrfs_qgroup_inherit()
> if the transaction handle corresponds to the one being used to enable the
> quotas.
>
> Fixes: 6426c7ad697d ("btrfs: qgroup: Fix qgroup accounting when creating
> snapshot")
> Signed-off-by: Filipe Manana <fdman...@suse.com>
> ---
> fs/btrfs/qgroup.c | 10 ++++++++--
> fs/btrfs/transaction.h | 1 +
> 2 files changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
> index d4917c0cddf5..3aec3bfa3d70 100644
> --- a/fs/btrfs/qgroup.c
> +++ b/fs/btrfs/qgroup.c
> @@ -908,6 +908,7 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
> trans = NULL;
> goto out;
> }
> + trans->enabling_quotas = true;
Should we put enabling_quotas bit into btrfs_transaction instead of
btrfs_trans_handle?
Isn't it possible to have different trans handle pointed to the same
transaction?
And I'm not really sure about the naming "enabling_quotas".
What about "quota_ioctl_mutex_hold"? (Well, this also sounds awful)
Thanks,
Qu
>
> fs_info->qgroup_ulist = ulist_alloc(GFP_KERNEL);
> if (!fs_info->qgroup_ulist) {
> @@ -2250,7 +2251,11 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle
> *trans, u64 srcid,
> u32 level_size = 0;
> u64 nums;
>
> - mutex_lock(&fs_info->qgroup_ioctl_lock);
> + if (trans->enabling_quotas)
> + lockdep_assert_held(&fs_info->qgroup_ioctl_lock);
> + else
> + mutex_lock(&fs_info->qgroup_ioctl_lock);
> +
> if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags))
> goto out;
>
> @@ -2413,7 +2418,8 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle
> *trans, u64 srcid,
> unlock:
> spin_unlock(&fs_info->qgroup_lock);
> out:
> - mutex_unlock(&fs_info->qgroup_ioctl_lock);
> + if (!trans->enabling_quotas)
> + mutex_unlock(&fs_info->qgroup_ioctl_lock);
> return ret;
> }
>
> diff --git a/fs/btrfs/transaction.h b/fs/btrfs/transaction.h
> index 703d5116a2fc..a5553a1dee30 100644
> --- a/fs/btrfs/transaction.h
> +++ b/fs/btrfs/transaction.h
> @@ -122,6 +122,7 @@ struct btrfs_trans_handle {
> bool reloc_reserved;
> bool sync;
> bool dirty;
> + bool enabling_quotas;
> struct btrfs_root *root;
> struct btrfs_fs_info *fs_info;
> struct list_head new_bgs;
>
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