Re: [PATCH V2] Btrfs: keep dropped roots in cache until transaction commit

["Holger Hoffstätte" <holger.hoffstaette@googlemail.com>]

On 09/15/15 16:07, Josef Bacik wrote:
> When dropping a snapshot we need to account for the qgroup changes.  If we drop
> the snapshot in all one go then the backref code will fail to find blocks from
> the snapshot we dropped since it won't be able to find the root in the fs root
> cache.  This can lead to us failing to find refs from other roots that pointed
> at blocks in the now deleted root.  To handle this we need to not remove the fs
> roots from the cache until after we process the qgroup operations.  Do this by
> adding dropped roots to a list on the transaction, and letting the transaction
> remove the roots at the same time it drops the commit roots.  This will keep all
> of the backref searching code in sync properly, and fixes a problem Mark was
> seeing with snapshot delete and qgroups.  Thanks,
> 
> Signed-off-by: Josef Bacik <jbacik@fb.com>
> ---
> V1->V2:
> -clear the trans tag when we are finished dropping the subvol so we don't try to
>   update the root during commit.

This V2 does indeed seem to fix the issues I reported with snapshot deletion &
concurrent sync. I've now created/filled/deleted countless snapshots while issuing
sync(s) in parallel, and the problem that I saw fairly frequently with V1 no longer
seems to occur here. Therefore:

Tested-by: Holger Hoffstätte <holger.hoffstaette@googlemail.com>

Thanks for the quick fix, Josef!

cheers
Holger