Re: barriers vs. reads

[Jamie Lokier <jamie@shareable.org>]

Jens Axboe wrote:
> > But do we have cases where reads must not cross write barriers ?
> 
> To me, it's the expected behaviour. If you issue a barrier write, a read
> issued later should not be able to fetch old data.

Two things:

   1. A read _which doesn't overlap writes before the barrier_
      should be ok before the barrier with no visible change.

      So, look at the block numbers and permit reordering if there's
      no overlap.  This reordering is semantically invisible.

   2. Other than O_DIRECT, can the I/O subsystem issue reads that
      overlap writes in flight?  Surely that never occurs?

      If it never occurs, then reads can be safely moved before write
      barriers without looking at block numbers.

-- Jamie