On Mon, 2014-03-24 at 10:30 +0000, David Laight wrote:
> From: Eric Dumazet
> > On Fri, 2014-03-21 at 14:52 -0400, David Miller wrote:
> > > From: Eric Dumazet <eric.duma...@gmail.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.
>
> Adding 'volatile' isn't sufficient to force gcc to write data
> into the area being checksummed.
You missed the point. Its not about forcing gcc to write data, because
it does.
Point is : gcc doesn't recompute the checksum a second time.
> ip_fast_csum() either needs an explicit "m" constraint for the actual
> buffer (and target) bytes, or the stronger "memory" constraint.
> The 'volatile' is then not needed.
What about you take a look at the actual code ?
"memory" constraint is already there. And no, its not enough, otherwise
I wouldn't have sent this mail.
I actually compiled the code and double checked.
0000000000007010 <foobar>:
7010: e8 00 00 00 00 callq 7015 <foobar+0x5>
7011: R_X86_64_PC32 __fentry__-0x4
7015: 55 push %rbp
7016: 31 c0 xor %eax,%eax
7018: b9 05 00 00 00 mov $0x5,%ecx
701d: 48 89 e5 mov %rsp,%rbp
7020: 48 83 ec 20 sub $0x20,%rsp
7024: 48 89 5d e8 mov %rbx,-0x18(%rbp)
7028: 4c 89 6d f8 mov %r13,-0x8(%rbp)
702c: 48 89 fb mov %rdi,%rbx
702f: 4c 89 65 f0 mov %r12,-0x10(%rbp)
7033: 41 89 d5 mov %edx,%r13d
7036: 66 89 47 0a mov %ax,0xa(%rdi)
703a: 66 89 77 02 mov %si,0x2(%rdi)
703e: 48 89 f8 mov %rdi,%rax
7041: 48 89 fe mov %rdi,%rsi
7044: 44 8b 20 mov (%rax),%r12d
7047: 83 e9 04 sub $0x4,%ecx
704a: 76 2e jbe 707a <foobar+0x6a>
704c: 44 03 60 04 add 0x4(%rax),%r12d
7050: 44 13 60 08 adc 0x8(%rax),%r12d
7054: 44 13 60 0c adc 0xc(%rax),%r12d
7058: 44 13 60 10 adc 0x10(%rax),%r12d
705c: 48 8d 40 04 lea 0x4(%rax),%rax
7060: ff c9 dec %ecx
7062: 75 f4 jne 7058 <foobar+0x48>
7064: 41 83 d4 00 adc $0x0,%r12d
7068: 44 89 e1 mov %r12d,%ecx
706b: 41 c1 ec 10 shr $0x10,%r12d
706f: 66 41 01 cc add %cx,%r12w
7073: 41 83 d4 00 adc $0x0,%r12d
7077: 41 f7 d4 not %r12d
707a: 31 c0 xor %eax,%eax
707c: 66 44 89 67 0a mov %r12w,0xa(%rdi)
7081: 48 c7 c7 00 00 00 00 mov $0x0,%rdi
7084: R_X86_64_32S .rodata.str1.1+0xabd
7088: e8 00 00 00 00 callq 708d <foobar+0x7d>
7089: R_X86_64_PC32 printk-0x4
708d: 66 44 89 6b 02 mov %r13w,0x2(%rbx)
7092: 66 44 89 63 0a mov %r12w,0xa(%rbx)
7097: 4c 8b 6d f8 mov -0x8(%rbp),%r13
709b: 48 8b 5d e8 mov -0x18(%rbp),%rbx
709f: 4c 8b 65 f0 mov -0x10(%rbp),%r12
70a3: c9 leaveq
70a4: c3 retq
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