Re: 4.0.0-rc4: panic in free_block

[David Miller <davem@davemloft.net>]

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 23 Mar 2015 12:47:49 -0700

> On Mon, Mar 23, 2015 at 12:08 PM, David Miller <davem@davemloft.net> 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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