On Mon, Nov 19, 2018 at 11:35 AM Qu Wenruo <quwenruo.bt...@gmx.com> wrote:
>
>
>
> On 2018/11/19 下午7:13, Filipe Manana wrote:
> > On Mon, Nov 19, 2018 at 11:09 AM Qu Wenruo <quwenruo.bt...@gmx.com> wrote:
> >>
> >>
> >>
> >> 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?
> >
> > Why?
> > Only the task which is enabling quotas needs to know about it.
>
> But it's the btrfs_qgroup_inherit() using the trans handler to avoid
> dead lock.
>
> What makes sure btrfs_qgroup_inherit() get the exactly same trans
> handler allocated here?
If it's the other task (the one creating a snapshot) that starts the
transaction commit,
it will have to wait for the task enabling quotas to release the
transaction - once that task
also calls commit_transaction(), it will skip doing the commit itself
and wait for the snapshot
one to finish the commit, while holding the qgroup mutex (this part I
missed before).
So yes we'll have to use a bit in the transaction itself instead.
>
> >
> >>
> >> Isn't it possible to have different trans handle pointed to the same
> >> transaction?
> >
> > Yes.
> >
> >>
> >> And I'm not really sure about the naming "enabling_quotas".
> >> What about "quota_ioctl_mutex_hold"? (Well, this also sounds awful)
> >
> > Too long.
>
> Anyway, current naming doesn't really show why we could skip mutex
> locking. Just hope to get some name better.
No name will ever show you that.
You'll always have to see where and how it's used, unless you want a
name like "dont_lock_mutex_because_we_locked_it_at_btrfs...".
>
> Thanks,
> Qu
>
> >
> >
> >>
> >> 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;
> >>>
> >>
>
