Re: [patch] lockdep: annotate mm/slab.c

[Christoph Lameter <clameter@sgi.com>]

On Thu, 13 Jul 2006, Arjan van de Ven wrote:

> [*] Note Note Note
> there is a corner case in the slab code that I personally don't trust at
> all. In the NUMA case, if the memory is not originally from your own
> node, the cache_free_alien() function takes, while having your own local
> lock, the lock of the remote node as well. (at least on my reading of
> the code) to free the memory to that node. I have yet to see where in
> the code it safeguards against that remote node doing the exact same
> thing in the opposite direction concurrently, and causing a basic ABBA
> deadlock.

Second look: I cannot find where we take our own local nodes list_lock.
We only take the lock from the remote node. Or is this related to the 
OFF_SLAB kfree issue?


We either have a alien cache structure established then:

We take a lock on the alien structure for node x from our own node 
(without holding our local list_lock!) and then we need take the remote 
list_lock for node x if the alien structure overflows and we then free
to the remote nodes list.

Or we do not have a alien cache structure established yet. Then:

We simply take the remote list_lock on node x and free directly to the 
foreign nodes list.



In an OFF_SLAB situation this may differ because then we call 
kmem_cache_free from slab_destroy. Ughhh... This looks extremely bad. 
Whew! We drop the list lock before calling slab_destroy.


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