Re: [PATCH 2/2] Btrfs: serialize flushers in reserve_metadata_bytes

David Sterba
Fri, 10 Jun 2011 17:47:59 -0000

On Thu, Jun 09, 2011 at 08:00:15PM +0200, David Sterba
wrote:
> On Thu, Jun 09, 2011 at 10:00:42AM -0400, Josef Bacik wrote:
> > We''re not trying to be perfect here, we''re trying to
be fast :).
>
> Be even faster with smp_rmb() :)

Arne made me think about this again. Let''s analyze it in more detail:

The read side, if(delalloc_bytes), utilizes a full barrier, which will
force all cpus to flush pending reads and writes. This will ensure
''if''
will see a fresh value.

However, there is no pairing write barrier and the write flush will
happen at some point in time, (delalloc_bytes += len), but completely
unsynchronized with the read side.

The smp_mb barrier has no desired synchonization effect. Moreover, it
has a performance hit.


Doing it right with barriers would mean to add smp_rmb before the
if(...) and smp_wmb after the "delalloc_bytes =", but only in the case
the variable is solely synchronized via barriers. Not our case. There is
the spinlock.

As strict correctness is not needed here, you admit that delalloc_bytes
might not correspond to the state of fs_info->delalloc_inodes and this
is not a problem. Fine. But then you do not need the smp_mb. The value
of delalloc_bytes will be "random" (ie. unsynchronized), with or
without
the barrier. Please drop it from the patch.


david