Re: [PATCH v5 2/2] buffer: record blockdev write errors in super_block that it backs

[Jeff Layton]

On Wed, 2020-04-15 at 16:06 +0200, Jan Kara wrote:
> On Wed 15-04-20 08:13:00, 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.
> > 
> > Since superblocks are RCU freed, hold the rcu_read_lock to ensure
> > that the superblock doesn't go away while we're marking it.
> > 
> > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > ---
> >  fs/buffer.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/fs/buffer.c b/fs/buffer.c
> > index f73276d746bb..2a4a5cc20418 100644
> > --- a/fs/buffer.c
> > +++ b/fs/buffer.c
> > @@ -1154,12 +1154,19 @@ EXPORT_SYMBOL(mark_buffer_dirty);
> >  
> >  void mark_buffer_write_io_error(struct buffer_head *bh)
> >  {
> > +	struct super_block *sb;
> > +
> >  	set_buffer_write_io_error(bh);
> >  	/* FIXME: do we need to set this in both places? */
> >  	if (bh->b_page && bh->b_page->mapping)
> >  		mapping_set_error(bh->b_page->mapping, -EIO);
> >  	if (bh->b_assoc_map)
> >  		mapping_set_error(bh->b_assoc_map, -EIO);
> > +	rcu_read_lock();
> > +	sb = bh->b_bdev->bd_super;
> 
> You still need READ_ONCE() here. Otherwise the dereference below can still
> result in refetch and NULL ptr deref.
> 
> 								Honza
> 

Huh? That seems like a really suspicious thing for the compiler/arch to
do. We are checking that sb isn't NULL before we dereference it. Doesn't
that imply a data dependency? How could the value of "sb" change after
that?

I'm also not sure I understand how using READ_ONCE really helps there if
we can't count on the value of a local variable not changing.

> > +	if (sb)
> > +		errseq_set(&sb->s_wb_err, -EIO);
> > +	rcu_read_unlock();
> >  }
> >  EXPORT_SYMBOL(mark_buffer_write_io_error);
> >  
> > -- 
> > 2.25.2
> > 

-- 
Jeff Layton <jlayton@xxxxxxxxxx>