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Date: Wed, 22 Feb 2017 09:54:02 +1100 From: Dave Chinner <david@...morbit.com> To: Peter Zijlstra <peterz@...radead.org> Cc: Elena Reshetova <elena.reshetova@...el.com>, linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org, gregkh@...uxfoundation.org, darrick.wong@...cle.com, Hans Liljestrand <ishkamiel@...il.com>, Kees Cook <keescook@...omium.org>, David Windsor <dwindsor@...il.com> Subject: Re: [PATCH 2/7] fs, xfs: convert xfs_buf.b_hold and xfs_buf.b_lru_ref from atomic_t to refcount_t 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 david@...morbit.com
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