Re: 4.0.0-rc4: panic in free_block

[David Ahern <david.ahern@oracle.com>]

On 3/20/15 6:47 PM, Linus Torvalds wrote:
>
>> Here's another data point: If I disable NUMA I don't see the problem.
>> Performance drops, but no NULL pointer splats which would have been panics.
>
> So the NUMA case triggers the per-node "n->shared" logic, which
> *should* be protected by "n->list_lock". Maybe there is some bug there
> - but since that code seems to do ok on x86-64 (and apparently older
> sparc too), I really would look at arch-specific issues first.

You raise a lot of valid questions and something to look into. But if 
the root cause were such a fundamental issue (CPU memory ordering, 
compiler bug, etc) why would it only occur on this one code path -- free 
with SLAB and NUMA -- and so consistently?

Continuing to poke around, but open to any suggestions. I have enabled 
every DEBUG I can find in the memory code and nothing is popping out. In 
terms of races wouldn't all the DEBUG checks affect timing? Yet, I am 
still seeing the same stack traces due to the same root cause.

David

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