RE: [PATCH 2/7] fs, xfs: convert xfs_buf.b_hold and xfs_buf.b_lru_ref from atomic_t to refcount_t

[Reshetova, Elena]

> On Tue, Feb 21, 2017 at 05:04:08PM +0100, Peter Zijlstra wrote:
> > On Tue, Feb 21, 2017 at 05:49:02PM +0200, Elena Reshetova wrote:
> > > @@ -1684,10 +1684,11 @@ xfs_buftarg_isolate(
> > >  	 * zero. If the value is already zero, we need to reclaim the
> > >  	 * buffer, otherwise it gets another trip through the LRU.
> > >  	 */
> > > -	if (!atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
> > > +	if (!refcount_read(&bp->b_lru_ref)) {
> > >  		spin_unlock(&bp->b_lock);
> > >  		return LRU_ROTATE;
> > >  	}
> > > +	refcount_dec_and_test(&bp->b_lru_ref);
> > >
> > >  	bp->b_state |= XFS_BSTATE_DISPOSE;
> > >  	list_lru_isolate_move(lru, item, dispose);
> >
> > This should never have passed testing.. refcount_dec_and_test() has a
> > __must_check.
> >
> > Furthermore the above seems to suggest thingies can live with a 0
> > refcount, so a straight conversion cannot work.
> 
> Yes, 0 is a valid value - the buffer lru reference is *not an object
> lifecycle reference count*. A value of zero means reclaim needs to
> take action if it sees that value - it does not mean that the object
> is not referenced by anyone (that's b_hold). i.e.  b_lru_ref is an
> "active reference weighting" used to provide a heirarchical reclaim
> bias toward less important metadata objects, and has no bearing on
> the actual active users of the object.


OK, so all of this suggests that we should not conver b_lru_ref to the refcount_t then. 
I will remove this conversion from this commit and only leave b_hold.

Thank you!

Best Regards,
Elena.