Re: [PATCH 2/2] xfs: take a ref on failed bufs in xfs_inode_item_push

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

On Mon, Nov 12, 2018 at 09:23:48AM -0500, Josef Bacik wrote:
> On Thu, Nov 08, 2018 at 10:37:40AM +1100, Dave Chinner wrote:
> > On Wed, Nov 07, 2018 at 03:10:55PM -0500, Josef Bacik wrote:
> > > If we failed to writeout a xfs_buf we'll grab a ref for it and put it on
> > > li->li_buf.  Then when submitting the failed bufs we'll clear LI_FAILED
> > > on the li, which clears the LI_FAILED flag, but also drops the ref on
> > > the buf.  Since it isn't on a IO list at this point this could very well
> > > be the last ref on the buf, which wreaks havoc when we go to add the buf
> > > to the delwrite list.  Fix this by holding a ref on the buf before we
> > > call xfs_buf_resubmit_failed_buffers in order to make sure the buf
> > > doesn't disappear before we're able to clear the error and add it to the
> > > delwri list.  This fixes the panics I was seeing with error injection.
> > > 
> > > Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> > > ---
> > >  fs/xfs/xfs_inode_item.c | 7 +++++--
> > >  1 file changed, 5 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/fs/xfs/xfs_inode_item.c b/fs/xfs/xfs_inode_item.c
> > > index fa1c4fe2ffbf..df49adf1989c 100644
> > > --- a/fs/xfs/xfs_inode_item.c
> > > +++ b/fs/xfs/xfs_inode_item.c
> > > @@ -503,13 +503,16 @@ xfs_inode_item_push(
> > >  	 * previously. Resubmit the buffer for IO.
> > >  	 */
> > >  	if (test_bit(XFS_LI_FAILED, &lip->li_flags)) {
> > > -		if (!xfs_buf_trylock(bp))
> > > +		xfs_buf_hold(bp);
> > > +		if (!xfs_buf_trylock(bp)) {
> > > +			xfs_buf_rele(bp);
> > >  			return XFS_ITEM_LOCKED;
> > > +		}
> > >  
> > >  		if (!xfs_buf_resubmit_failed_buffers(bp, buffer_list))
> > >  			rval = XFS_ITEM_FLUSHING;
> > >  
> > > -		xfs_buf_unlock(bp);
> > > +		xfs_buf_relse(bp);
> > >  		return rval;
> > >  	}
> > 
> > This just doesn't smell right to me.
> > 
> > /me rummages around in the code
> > 
> > Ok, so I think the bug is in xfs_buf_resubmit_failed_buffers() in
> > that it removes all the "failed" reference counts from the buffer
> > before it adds the delwri reference count back to the buffer. Taking
> > a high level reference count like above just papers over the
> > transient reference counting error in
> > xfs_buf_resubmit_failed_buffers().
> > 
> > It also fails to fix the other xfs_buf_resubmit_failed_buffers()
> > caller, which has exactly the same problem (dquot writeback).
> > 
> > Perhaps something like the patch below?
> > 
> > Cheers,
> > 
> > Dave.
> > -- 
> > Dave Chinner
> > david@fromorbit.com
> > 
> > 
> > xfs: fix transient reference count error in xfs_buf_resubmit_failed_buffers
> > 
> > From: Dave Chinner <dchinner@redhat.com>
> > 
> > When retrying a failed inode or dquot buffer,
> > xfs_buf_resubmit_failed_buffers() clears all the failed flags from
> > the inode/dquot log items. In doing so, it also drops all the
> > reference counts on the buffer that the failed log items hold. This
> > means it can drop all the active references on the buffer and hence
> > free the buffer before it queues it for write again.
> > 
> > Putting the buffer on the delwri queue takes a reference to the
> > buffer (so that it hangs around until it has been written and
> > completed), but this goes bang if the buffer has already been freed.
> > 
> > Hence we need to add the buffer to the delwri queue before we remove
> > the failed flags from the log items attached to the buffer to ensure
> > it always remains referenced during the resubmit process.
> > 
> > Reported-by: Josef Bacik <josef@toxicpanda.com>
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> 
> Dave,
> 
> Are you planning on sending this along as is?  I'm going to throw it in our
> kernel if you are happy with it.  Thanks,

I'd appreciate it if this patch could be sent to the mailing list in its
own thread to avoid "new patch buried in other thread" syndrome.

--D

> Josef