Re: 2.6.14-rc1-git-now still dying in mm/slab - this time line 1849

[Andrew Morton]

Christoph Lameter <clameter@xxxxxxxxxxxx> wrote:
>
> On Mon, 19 Sep 2005, Andrew Morton wrote:
> 
> > 	list_for_each(walk, &cache_chain) {
> > 		kmem_cache_t *searchp;
> > 		struct list_head* p;
> > 		int tofree;
> > 		struct slab *slabp;
> > 
> > 		searchp = list_entry(walk, kmem_cache_t, next);
> > 
> > 		if (searchp->flags & SLAB_NO_REAP)
> > 			goto next;
> > 
> > 		check_irq_on();
> > 
> > 		l3 = searchp->nodelists[numa_node_id()];
> > 		if (l3->alien)
> > 			drain_alien_cache(searchp, l3);
> > ->preempt here
> > 		spin_lock_irq(&l3->list_lock);
> > 
> > 		drain_array_locked(searchp, ac_data(searchp), 0,
> > 				numa_node_id());
> > ->oops, wrong node.
> 
> This is called from keventd which exists per processor. Hmmm... This looks 
> as if it can change processors after all

Well no, it would be a big bug if a keventd thread were to change CPUs.

It's OK to rely upon the pinnedness of keventd I guess - a comment would be
nice.

> but the slab allocator depends on 
> it running on the right processor. So does the page allocator. sigh. What 
> is the point of having per processor workqueues if they do not stay on 
> the assigned processor?

They do.  I don't believe that preemption is the source of this BUG. 
(Petr, does CONFIG_PREEMPT=n fix it?)

> The fast fix for this case is to get the node number once and then use it 
> consistently.

If one is writing preempt-safe code then one should disable preemption
before copying the current CPU number into a local variable.

> But we really need to audit the slab and page allocator for 
> additional cases like this or disable preempt and check for the right 
> processor in cache_reap().

numa_node_id() must use smp_processor_id(), not raw_smp_processor_id(). 
Then all the runtime squawks need to be audited and fixed, or switched to
(new) raw_numa_node_id() if is is verified that a CPU/node switch at any
time is OK.


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