Re: [PATCH 3/4] xfs: make xfs_inode_set_eofblocks_tag cheaper for the common case

[Dave Chinner <david@xxxxxxxxxxxxx>]

On Tue, Aug 30, 2016 at 04:40:06PM +0200, Christoph Hellwig wrote:
> On Fri, Aug 26, 2016 at 12:02:09PM -0400, Brian Foster wrote:
> > > I don't think taking it should be too bad, but given the ops ordering
> > > it also seems entirely pointless to even take it.
> > > 
> > 
> > Then why are we taking it? I assumed it at least served as a memory
> > barrier...
> 
> I meant to take it for that early check, not in general.
> 
> I guess this is another hint we should try to look into using proper
> atomic bitops here..

I think we've looked at that in the past, but there were cases where
we have to do things atomically with setting/clearing the flags and
that required the spinlock to protect the flag modifications as
well. IIRC there are also cases where we have to check/set multiple
flags at once, which we cannot do with atomic bit ops.

Perhaps the code has changed enough that there isn't a problem
anymore, but I don't think that is the case...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx