On Mon, 9 May 2016 09:33:25 -0700 Alexander Duyck <alexander.du...@gmail.com> wrote:
> On Mon, May 9, 2016 at 6:44 AM, Jesper Dangaard Brouer > <bro...@redhat.com> wrote: > > It have always been required to call napi_alloc_skb from NAPI/softirq > > context, which implies running with local_bh_disable'ed. Thus, this > > code path should already be well tested. But recent SKB bulk changes > > introduced will make this more volatile and bugs more subtle, if this > > is violated. > > It hasn't been required to be in the softirq routine, bottom halves > could have been disabled. I used the NAPI parameter to try and > enforce a strong correlation between the two but I didn't make it a > hard requirement. It isn't until you try to do the recycling that it > becomes more of a requirement because then it is tied into the bulk > free. > > The bulk free routine had a stronger requirement for NAPI because it > needed to have the __kfree_skb_flush routine called to finally free > the skbuffs that were still hanging off the percpu skbuff structure. > > > To catch any driver violating this add a loud WARN_ON. > > > > Performance wise, I do worry about adding this runtime check code into > > the hotpath, of this highly optimized function call. I've > > micro-benchmarked it with both IP-forwarding and local UDP delivery, > > and didn't see any regressions. It does adds extra code size (icache). > > > > add/remove: 0/0 grow/shrink: 1/0 up/down: 43/0 (43) > > function old new delta > > __napi_alloc_skb 461 504 +43 > > > > Signed-off-by: Jesper Dangaard Brouer <bro...@redhat.com> > > --- > > net/core/skbuff.c | 3 +++ > > 1 file changed, 3 insertions(+) > > > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c > > index e85f1065b263..99addbf66f2e 100644 > > --- a/net/core/skbuff.c > > +++ b/net/core/skbuff.c > > @@ -498,6 +498,9 @@ struct sk_buff *__napi_alloc_skb(struct napi_struct > > *napi, unsigned int len, > > struct sk_buff *skb; > > void *data; > > > > + /* Catch drivers violating, not having local BH disabled */ > > + WARN_ON(!in_softirq()); > > + > > I'm pretty sure this still lets you get away with just disabling > bottom halves. I think what you would want is in_serving_softirq(). No, we want to allow this being call with just disabled bottom halves. Thus, we have to use in_softirq() and not in_serving_softirq(). > > len += NET_SKB_PAD + NET_IP_ALIGN; > > > > if (unlikely((len > SKB_WITH_OVERHEAD(PAGE_SIZE)) || > > -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat Author of http://www.iptv-analyzer.org LinkedIn: http://www.linkedin.com/in/brouer
