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.
Cheers,
Dave.
--
Dave Chinner
da...@fromorbit.com
