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

[Jan Kara]

On Wed 15-04-20 12:22:27, Jeff Layton wrote:
> 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?

Because the compiler is free to optimize the local variable away and
actually compile the dereference below as bh->b_bdev->bd_super->s_wb_err
(from C11 standard POV such code is equivalent since in C11 memory model
it is assumed there are no concurrent accesses). And READ_ONCE() is a way
to forbid compiler from doing such optimization - through 'volatile'
keyword it tells the compiler there may be concurrent accesses happening
and makes sure the value is really fetched into the local variable and used
from there. There are good articles about this on LWN - I'd give you a link
but LWN seems to be down today. But the latest article is about KCSAN and
from there are links to older articles about compiler optimizations.

> 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.

I hope I've explained this above.

								Honza

> > > +	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>
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR