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Date: Sun, 23 Mar 2014 19:50:10 -0700 From: Eric Dumazet <eric.dumazet@...il.com> To: David Miller <davem@...emloft.net> Cc: andi@...stfloor.org, hpa@...or.com, kaber@...sh.net, herbert@...dor.apana.org.au, hkchu@...gle.com, mwdalton@...gle.com, netdev@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [RFC] csum experts, csum_replace2() is too expensive On Fri, 2014-03-21 at 14:52 -0400, David Miller wrote: > From: Eric Dumazet <eric.dumazet@...il.com> > Date: Fri, 21 Mar 2014 05:50:50 -0700 > > > It looks like a barrier() would be more appropriate. > > barrier() == __asm__ __volatile__(:::"memory") Indeed, but now you mention it, ip_fast_csum() do not uses volatile keyword on x86_64, and has no "m" constraint either. This means that for the following hypothetical networking code : void foobar(struct iphdr *iph, __be16 newlen, __be16 newid) { iph->tot_len = newlen; iph->check = 0; iph->check = ip_fast_csum((u8 *)iph, 5); pr_err("%p\n", iph); iph->id = newid; iph->check = 0; iph->check = ip_fast_csum((u8 *)iph, 5); } ip_fast_csum() is done _once_ only. Following patch seems needed. Thats one another call for x86 code factorization ... diff --git a/arch/x86/include/asm/checksum_64.h b/arch/x86/include/asm/checksum_64.h index e6fd8a026c7b..c67778544880 100644 --- a/arch/x86/include/asm/checksum_64.h +++ b/arch/x86/include/asm/checksum_64.h @@ -46,7 +46,7 @@ static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl) { unsigned int sum; - asm(" movl (%1), %0\n" + asm volatile(" movl (%1), %0\n" " subl $4, %2\n" " jbe 2f\n" " addl 4(%1), %0\n" -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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