Christoph Lameter <[email protected]> wrote:
>
> On Mon, 19 Sep 2005, Andrew Morton wrote:
>
> > Well. The CPU_UP_CANCELED locking in cpuup_callback() looks borked to me -
> > it takes cachep->nodelists[node]->list_lock and then calls
> > drain_alien_cache() which appears to take the same lock. But that's not
> > the problem here.
> >
> > The code in cache_reap() recalculates numa_node_id() multiple times, so if
> > the caller changes CPUs then this assertion will trigger. However it's
> > running under keventd here, which is pinned to a single CPU. Still, it
> > would be useful if you could try putting preempt_disable()s in
> > cache_reap(), or change cache_reap() to evaluate numa_node_id() just the
> > once, and cache that in a local variable.
>
> drain_array_cache_locked calls check_spinlock_acquired_node which is in
> turn insuring that interrupts are off. So no move to a different processor
> should be possible.
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.
Still, this should all be pinned to one CPU, by happenstance.
> However, that is contradicted by __wake_up calling
> drain_array_cache_locked. The process just woke up?
Not sure what you mean here.
> > I wonder why numa_node_id() uses raw_smp_processor_id()? That's just
> > asking for preempt non-atomicity bugs.
>
> Accessing arrays indexed by node number even works if the process
> continues to be executed on another node.
That's a special case and the callers should be changed to use a new
raw_numa_node_id() in that case.
Code which calls numa_node_id() and then continues to use the result of
that in preemptible code is often buggy. Code which reevaluates
numa_node_id() in preemptible code and assumes that it returned the same
thing is even buggier (unless it happens to be CPU pinned).
numa_node_id() is doing a bad thing and should be converted to use
smp_processor_id() so we can identify all the possibly-buggy callsites.
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