Hi guys, We found this issue when we run mbuf_autotest. It failed on a aarch64 platform. I am not sure if it can be reproduced on other platforms. Regards, Jie Liu
-----Original Message----- From: Ananyev, Konstantin [mailto:konstantin.anan...@intel.com] Sent: 2017年10月13日 1:06 To: Olivier MATZ <olivier.m...@6wind.com>; Jia He <hejia...@gmail.com> Cc: dev@dpdk.org; He, Jia <jia...@hxt-semitech.com>; Liu, Jie2 <jie2....@hxt-semitech.com>; Zhao, Bing <bing.z...@hxt-semitech.com>; jerin.ja...@caviumnetworks.com Subject: RE: [PATCH] ring: guarantee ordering of cons/prod loading when doing enqueue/dequeue Hi guys, > -----Original Message----- > From: Olivier MATZ [mailto:olivier.m...@6wind.com] > Sent: Thursday, October 12, 2017 4:54 PM > To: Jia He <hejia...@gmail.com> > Cc: dev@dpdk.org; jia...@hxt-semitech.com; jie2....@hxt-semitech.com; > bing.z...@hxt-semitech.com; Ananyev, Konstantin > <konstantin.anan...@intel.com>; jerin.ja...@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 <hejia...@gmail.com> > > Signed-off-by: jia...@hxt-semitech.com > > Signed-off-by: jie2....@hxt-semitech.com > > Signed-off-by: bing.z...@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 This email is intended only for the named addressee. It may contain information that is confidential/private, legally privileged, or copyright-protected, and you should handle it accordingly. If you are not the intended recipient, you do not have legal rights to retain, copy, or distribute this email or its contents, and should promptly delete the email and all electronic copies in your system; do not retain copies in any media. If you have received this email in error, please notify the sender promptly. Thank you.