Re: raid6 badness

["H. Peter Anvin" <hpa@zytor.com>]

Followup to:  <Pine.LNX.4.58.0401301158340.8900@sapphire.newearth.org>
By author:    "Michael V. David" <michael@mvdavid.com>
In newsgroup: linux.dev.kernel
>
> This x86_64 system has dual Opteron CPUs on a Tyan 2880 board. Kernel
> version string:
> 
> Linux version 2.6.2-bk4 (michael@sapphire) (gcc version 3.3.2 20040119 (Red Hat Linux 3.3.2-8)) #3 SMP Fri Jan 30
> 08:56:11 EST 2004
> 
> The same problem was produced with kernel versions 2.6.2-rc2 and
> 2.6.2-rc2-bk4. Output reproduced here is from -bk4.
> 
> If raid6 is compiled into the kernel, the kernel panics while
> starting. In the present case, it was compiled as a module. On
> loading, there is a segfault, and syslog gets what follows:
> 
> ---<snip>---
> raid6: int64x1   1175 MB/s
> raid6: int64x2   1734 MB/s
> raid6: int64x4   1773 MB/s
> raid6: int64x8   1273 MB/s
> general protection fault: 0000 [1]
> CPU 1
> Pid: 7310, comm: modprobe Not tainted
> RIP: 0010:[<ffffffffa0186383>] <ffffffffa0186383>{:raid6:raid6_sse21_gen_syndrome+51}
> RSP: 0018:0000010021825dd8  EFLAGS: 00010202
                           ^
                           
It crashes because the stack is misaligned.  x86-64 requires that the
stack is always aligned to a 16-byte boundary, but your stack pointer
isn't.

The RAID-6 code for x86-64 specifically assumes proper stack
alignment, so a misaligned stack is fatal.

I don't know what would cause the stack to be misaligned, however.

	-hpa