[dpdk-dev] [PATCH] ring: guarantee ordering of cons/prod loading when doing enqueue/dequeue

[Ananyev, Konstantin]

Hi guys,

> -----Original Message-----
> From: Olivier MATZ [mailto:olivier.matz at 6wind.com]
> Sent: Thursday, October 12, 2017 4:54 PM
> To: Jia He <hejianet at gmail.com>
> Cc: dev at dpdk.org; jia.he at hxt-semitech.com; jie2.liu at hxt-semitech.com; bing.zhao at hxt-semitech.com; Ananyev, Konstantin
> <konstantin.ananyev at intel.com>; jerin.jacob at caviumnetworks.com
> Subject: Re: [PATCH] ring: guarantee ordering of cons/prod loading when doing enqueue/dequeue
> 
> Hi,
> 
> On Tue, Oct 10, 2017 at 05:56:36PM +0800, Jia He wrote:
> > Before this patch:
> > In __rte_ring_move_cons_head()
> > ...
> >         do {
> >                 /* Restore n as it may change every loop */
> >                 n = max;
> >
> >                 *old_head = r->cons.head;                //1st load
> >                 const uint32_t prod_tail = r->prod.tail; //2nd load
> >
> > In weak memory order architectures(powerpc,arm), the 2nd load might be
> > reodered before the 1st load, that makes *entries is bigger than we wanted.
> > This nasty reording messed enque/deque up.
> >
> > cpu1(producer)          cpu2(consumer)          cpu3(consumer)
> >                         load r->prod.tail
> > in enqueue:
> > load r->cons.tail
> > load r->prod.head
> >
> > store r->prod.tail
> >
> >                                                 load r->cons.head
> >                                                 load r->prod.tail
> >                                                 ...
> >                                                 store r->cons.{head,tail}
> >                         load r->cons.head
> >
> > THEN,r->cons.head will be bigger than prod_tail, then make *entries very big
> >
> > After this patch, the old cons.head will be recaculated after failure of
> > rte_atomic32_cmpset
> >
> > There is no such issue in X86 cpu, because X86 is strong memory order model
> >
> > Signed-off-by: Jia He <hejianet at gmail.com>
> > Signed-off-by: jia.he at hxt-semitech.com
> > Signed-off-by: jie2.liu at hxt-semitech.com
> > Signed-off-by: bing.zhao at hxt-semitech.com
> >
> > ---
> >  lib/librte_ring/rte_ring.h | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/lib/librte_ring/rte_ring.h b/lib/librte_ring/rte_ring.h
> > index 5e9b3b7..15c72e2 100644
> > --- a/lib/librte_ring/rte_ring.h
> > +++ b/lib/librte_ring/rte_ring.h
> > @@ -409,6 +409,10 @@ __rte_ring_move_prod_head(struct rte_ring *r, int is_sp,
> >  		n = max;
> >
> >  		*old_head = r->prod.head;
> > +
> > +		/* load of prod.tail can't be reordered before cons.head */
> > +		rte_smp_rmb();
> > +
> >  		const uint32_t cons_tail = r->cons.tail;
> >  		/*
> >  		 *  The subtraction is done between two unsigned 32bits value
> > @@ -517,6 +521,10 @@ __rte_ring_move_cons_head(struct rte_ring *r, int is_sc,
> >  		n = max;
> >
> >  		*old_head = r->cons.head;
> > +
> > +		/* load of prod.tail can't be reordered before cons.head */
> > +		rte_smp_rmb();
> > +
> >  		const uint32_t prod_tail = r->prod.tail;
> >  		/* The subtraction is done between two unsigned 32bits value
> >  		 * (the result is always modulo 32 bits even if we have
> > --
> > 2.7.4
> >
> 
> The explanation convinces me.
> 
> However, since it's in a critical path, it would be good to have other
> opinions. This patch reminds me this discussion, that was also related to
> memory barrier, but at another place:
> http://dpdk.org/ml/archives/dev/2016-July/043765.html
> Lead to that patch: http://dpdk.org/browse/dpdk/commit/?id=ecc7d10e448e
> But finally reverted: http://dpdk.org/browse/dpdk/commit/?id=c3acd92746c3
> 
> Konstatin, Jerin, do you have any comment?

For IA, as rte_smp_rmb() is just a compiler_barrier, that patch shouldn't make any difference,
but  I can't see how read reordering would screw things up here...
Probably just me and arm or ppc guys could explain what will be the problem
if let say cons.tail will be read before prod.head in __rte_ring_move_prod_head().
I wonder Is there a simple test-case to reproduce that problem (on arm or ppc)?
Probably new test-case for rte_ring autotest is needed, or is it possible to reproduce
it with existing one?  
Konstantin