Re: [PATCH v2 1/4] Btrfs: fix wrong send_in_progress accounting

[David Sterba <dsterba@suse.cz>]

On Wed, Jan 08, 2014 at 11:09:02PM +0800, Wang Shilong wrote:
> 
> Hello David,
> 
> > On Tue, Jan 07, 2014 at 05:25:18PM +0800, Wang Shilong wrote:
> >> Steps to reproduce:
> >> # mkfs.btrfs -f /dev/sda8
> >> # mount /dev/sda8 /mnt
> >> # btrfs sub snapshot -r /mnt /mnt/snap1
> >> # btrfs sub snapshot -r /mnt /mnt/snap2
> >> # btrfs send /mnt/snap1 -p /mnt/snap2 -f /mnt/1
> >> # dmesg
> >> 
> >> The problem is that we will sort clone roots(include @send_root), it
> >> might push @send_root before thus @send_root's @send_in_progress will
> >> be decreased twice.
> > 
> > Of course, the sort(). I think your fix adds some complexity that's not
> > necessary. Whether the clone_roots array is sorted is not important, we
> > just have to process each root once.
> > 
> > send_root becomes a clone_root member, so the missing part is to account
> > in the rollback counter:
> > 
> > --- a/fs/btrfs/send.c
> > +++ b/fs/btrfs/send.c
> > @@ -4937,6 +4937,7 @@ long btrfs_ioctl_send(struct file *mnt_file, void __user *arg_)
> >         * for possible clone sources.
> >         */
> >        sctx->clone_roots[sctx->clone_roots_cnt++].root = sctx->send_root;
> > +       clone_sources_to_rollback++;
> 
> Not really, If we fail to come here, we still need decrease @send_root.

Right. I was thinking if the code can be simplified somehow, but don't
have anything vastly better. Can you please add a comment to the first
branch that send_root is processed in the loop and not missed? It looks
unabalanced when it's handled just a few lines below and not in the 1st
loop.

thanks,
david