Re: [dpdk-dev] [PATCH] mbuf: add comment explaining confusing code

[Neil Horman <nhorman@tuxdriver.com>]

On Fri, Mar 27, 2015 at 11:32:38AM +0000, Bruce Richardson wrote:
> On Fri, Mar 27, 2015 at 06:29:56AM -0400, Neil Horman wrote:
> > On Thu, Mar 26, 2015 at 09:14:54PM +0000, Bruce Richardson wrote:
> > > The logic used in the condition check before freeing an mbuf is
> > > sometimes confusing, so explain it in a proper comment.
> > > 
> > > Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
> > > ---
> > >  lib/librte_mbuf/rte_mbuf.h | 10 ++++++++++
> > >  1 file changed, 10 insertions(+)
> > > 
> > > diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> > > index 17ba791..0265172 100644
> > > --- a/lib/librte_mbuf/rte_mbuf.h
> > > +++ b/lib/librte_mbuf/rte_mbuf.h
> > > @@ -764,6 +764,16 @@ __rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
> > >  {
> > >  	__rte_mbuf_sanity_check(m, 0);
> > >  
> > > +	/*
> > > +	 * Check to see if this is the last reference to the mbuf.
> > > +	 * Note: the double check here is deliberate. If the ref_cnt is "atomic"
> > > +	 * the call to "refcnt_update" is a very expensive operation, so we
> > > +	 * don't want to call it in the case where we know we are the holder
> > > +	 * of the last reference to this mbuf i.e. ref_cnt == 1.
> > > +	 * If however, ref_cnt != 1, it's still possible that we may still be
> > > +	 * the final decrementer of the count, so we need to check that
> > > +	 * result also, to make sure the mbuf is freed properly.
> > > +	 */
> > >  	if (likely (rte_mbuf_refcnt_read(m) == 1) ||
> > >  			likely (rte_mbuf_refcnt_update(m, -1) == 0)) {
> > >  
> > > -- 
> > > 2.1.0
> > > 
> > > 
> > 
> > NAK
> >  the comment is incorrect, a return code of 1 from rte_mbuf_refcnt_read doesn't
> > guarantee you are the last holder of the buffer if two contexts have a pointer
> > to it.
> If two threads have pointers to it, and are both going to free it, the refcnt
> must be 2 not one, otherwise the refcnt is meaningless.
> 
Please see my note in zoltans thread.  I illustrate why its important to fix
this in several ways.  Relying of every context to have a refcnt on an object
before having  pointer reference to it is impossible to do without additional
locking.


> /Bruce
> 
> > 
> > Zoltans patch is the correct solution here, expensive or not.  I wrote up my
> > explination in this thread:
> > http://dpdk.org/ml/archives/dev/2015-March/015839.html
> > 
> > 
>