Hi, On 07/11/2016 01:22 PM, Kuusisaari, Juhamatti wrote: > > Hi, > >>> -----Original Message----- >>> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Juhamatti >>> Kuusisaari >>> Sent: Monday, July 11, 2016 11:21 AM >>> To: dev at dpdk.org >>> Subject: [dpdk-dev] [PATCH] lib: move rte_ring read barrier to correct >>> location >>> >>> Fix the location of the rte_ring data dependency read barrier. >>> It needs to be called before accessing indexed data to ensure that the >>> data itself is guaranteed to be correctly updated. >>> >>> See more details at kernel/Documentation/memory-barriers.txt >>> section 'Data dependency barriers'. >> >> >> Any explanation why? >> From my point smp_rmb()s are on the proper places here :) Konstantin > > The problem here is that on a weak memory model system the CPU is > allowed to load the address data out-of-order in advance. > If the read barrier is after the DEQUEUE, you might end up having the old > data there on a race situation when the buffer is continuously full. > Having it before the DEQUEUE guarantees that the load is not done > in advance. > > On Intel, it should not matter due to different memory model, so this is > limited to weak memory model systems.
I agree with Juhamatti. To me, the reading of consumer_head must occur before the reading of objects ptrs. That was the case before, and this is something I already noticed when I sent that mail: http://dpdk.org/ml/archives/dev/2014-March/001742.html At that time, only Intel CPUs were supported, so it did not make any difference. Juhamatti, do you have a setup where you can trigger the issue or is it something you've seen by code review? Thanks, Olivier
