Re: 4.0.0-rc4: panic in free_block
From: David Miller
Date: Mon Mar 23 2015 - 15:52:42 EST
From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Date: Mon, 23 Mar 2015 12:47:49 -0700
> On Mon, Mar 23, 2015 at 12:08 PM, David Miller <davem@xxxxxxxxxxxxx> wrote:
>>
>> Sure you could do that in C, but I really want to avoid using memcpy()
>> if dst and src overlap in any way at all.
>>
>> Said another way, I don't want to codify that "64" thing. The next
>> chip could do 128 byte initializing stores.
>
> But David, THAT IS NOT WHAT YOUR BROKEN ASM DOES ANYWAY!
>
> Read it again. Your asm code does not check for overlap. Look at this:
>
> cmp %o0, %o1
> bleu,pt %xcc, 2f
>
> and ponder. It's wrong.
Right, it's not checking for overlap. It's checking for "does a
forward copy work?"
That's the standard test for this, and it's what glibc uses in it's
generic memmove() implementation FWIW. (granted, I know glibc is not
generally a good source for "right way to do things :-)
> The new asm version is better than the old one, because the new
> breakage is about really bad performance rather than actively
> breaking, but still..
I accept that it's suboptimal.
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