Re: [PATCH 14/20] block: remove the nr_sects field in struct hd_struct

[Jan Kara <jack@xxxxxxx>]

On Fri 20-11-20 15:59:56, Matthew Wilcox wrote:
> On Fri, Nov 20, 2020 at 04:32:53PM +0100, Christoph Hellwig wrote:
> > On Fri, Nov 20, 2020 at 12:21:21PM +0100, Jan Kara wrote:
> > > > > AFAICT bd_size_lock is pointless after these changes so we can just 
> > > > > remove
> > > > > it?
> > > > 
> > > > I don't think it is, as reuqiring bd_mutex for size updates leads to
> > > > rather awkward lock ordering problems.
> > > 
> > > OK, let me ask differently: What is bd_size_lock protecting now? Ah, I 
> > > see,
> > > on 32-bit it is needed to prevent torn writes to i_size, right?
> > 
> > Exactly.  In theory we could skip it for 64-bit, but as updating the
> > size isn't a fast path, and struct block_device isn't super size critical
> > I'd rather keep the same code for 32 vs 64-bit builds.
> 
> Is it better to switch to i_size_write() / i_size_read()?

The code is already switched to it AFAICT (the lock is really only used in
the two places that write i_size). But the problem is that in theory two
i_size_write() calls can race in a way that the resulting stored i_size is a
mix of two stored sizes. Now I have hard time imagining how this could
happen for a block device and if two reconfigurations of a block device
could race like that we'd have a large problems anyway...

                                                                Honza

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