Re: [PATCH v4 RESEND 2/2] buffer: record blockdev write errors in super_block that it backs

[Jeff Layton]

On Tue, 2020-04-14 at 18:26 +0200, Jan Kara wrote:
> On Tue 14-04-20 08:04:09, Jeff Layton wrote:
> > From: Jeff Layton <jlayton@xxxxxxxxxx>
> > 
> > When syncing out a block device (a'la __sync_blockdev), any error
> > encountered will only be recorded in the bd_inode's mapping. When the
> > blockdev contains a filesystem however, we'd like to also record the
> > error in the super_block that's stored there.
> > 
> > Make mark_buffer_write_io_error also record the error in the
> > corresponding super_block when a writeback error occurs and the block
> > device contains a mounted superblock.
> > 
> > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> 
> The patch looks good to me. I'd just note that bh->b_bdev->bd_super
> dereference is safe only because we will flush all dirty data when
> unmounting a filesystem which is somewhat tricky. Maybe that warrants a
> comment? Otherwise feel free to add:
> 
> Reviewed-by: Jan Kara <jack@xxxxxxx>

Oh, hmm...now that I look again, I'm not sure this is actually safe.

bh->b_bdev gets cleared out as we discard the buffer, so I don't think
that could end up getting zeroed while we're still using it.

The bd_super pointer gets zeroed out in kill_block_super, and after that
point it calls sync_blockdev(). Could writeback error processing race
with kill_block_super such that bd_inode gets set to NULL after we test
it but before we dereference it?

Thanks,
-- 
Jeff Layton <jlayton@xxxxxxxxxx>