Re: [PATCH] xfs_repair: coordinate parallel updates to the rt bitmap

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Sep 24, 2020 at 06:40:41AM +0100, Christoph Hellwig wrote:
> On Wed, Sep 23, 2020 at 11:24:37AM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > Actually take the rt lock before updating the bitmap from multiple
> > threads.  This fixes an infrequent corruption problem when running
> > generic/013 and rtinherit=1 is set on the root dir.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> >  repair/dinode.c |    4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/repair/dinode.c b/repair/dinode.c
> > index 57013bf149b2..07f3f83aef8c 100644
> > --- a/repair/dinode.c
> > +++ b/repair/dinode.c
> > @@ -184,6 +184,7 @@ process_rt_rec(
> >  	xfs_rfsblock_t		*tot,
> >  	int			check_dups)
> >  {
> > +	struct aglock		*lock = &ag_locks[(signed)NULLAGNUMBER];
> 
> Err, what is this weird cast doing here?

Well.... ag_locks is allocated with length ag_locks[agcount + 1], and
then the pointer is incremented so that ag_locks[-1] is the rt lock.

NULLAGNUMBER is cast to xfs_agnumber_t, which is uint32_t, so we have to
cast it back to signed so that we actually do the pointer subtraction(!)

Yeah, I know, it's nuts...

--D

> The rest looks sane.