Messages in this thread | | | From | Menglong Dong <> | Date | Fri, 19 Aug 2022 23:17:04 +0800 | Subject | Re: [PATCH net-next v4] net: skb: prevent the split of kfree_skb_reason() by gcc |
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On Fri, Aug 19, 2022 at 1:09 AM Jakub Kicinski <kuba@kernel.org> wrote: > > On Tue, 16 Aug 2022 11:28:46 +0800 menglong8.dong@gmail.com wrote: > > From: Menglong Dong <imagedong@tencent.com> > > [...] > > Sorry for a late and possibly off-topic chime in, is the compiler > splitting it because it thinks that skb_unref() is going to return > true? I don't think that's the likely case, so maybe we're better > off wrapping that skb_unref() in unlikely()?
I think your thought is totally right, considering the instruction that I disassembled:
ffffffff819fea20 <kfree_skb_reason>: ffffffff819fea20: e8 cb 2c 40 00 call ffffffff81e016f0 <__fentry__> ffffffff819fea25: 48 85 ff test %rdi,%rdi ffffffff819fea28: 74 25 je ffffffff819fea4f <kfree_skb_reason+0x2f> ffffffff819fea2a: 8b 87 d4 00 00 00 mov 0xd4(%rdi),%eax /* this is just the instruction that compiled from skb_unref() */ ffffffff819fea30: 83 f8 01 cmp $0x1,%eax ffffffff819fea33: 75 0b jne ffffffff819fea40 <kfree_skb_reason+0x20> ffffffff819fea35: 55 push %rbp ffffffff819fea36: 48 89 e5 mov %rsp,%rbp ffffffff819fea39: e8 42 ff ff ff call ffffffff819fe980 <kfree_skb_reason.part.0> ffffffff819fea3e: 5d pop %rbp ffffffff819fea3f: c3 ret ffffffff819fea40: f0 ff 8f d4 00 00 00 lock decl 0xd4(%rdi) ffffffff819fea47: 0f 88 e5 44 27 00 js ffffffff81c72f32 <__noinstr_text_end+0x255d> ffffffff819fea4d: 74 e6 je ffffffff819fea35 <kfree_skb_reason+0x15> ffffffff819fea4f: c3 ret
The compiler just splits the code after skb_unref() to another. After I warp the skb_unref() in unlinkly(), this function is not splitted any more.
Yeah, I think we can make skb_unref() wrapped by unlikely() by the way.
Thanks! Menglong Dong
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