Messages in this thread | | | From | "Reshetova, Elena" <> | Subject | RE: [PATCH 2/7] fs, xfs: convert xfs_buf.b_hold and xfs_buf.b_lru_ref from atomic_t to refcount_t | Date | Wed, 22 Feb 2017 11:15:09 +0000 |
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> 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.
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