Re: [PATCH 0/6] stackdepot, kasan, workqueue: Avoid expanding stackdepot slabs when holding raw_spin_lock

[Sebastian Andrzej Siewior]

On 2021-09-10 12:50:51 [+0200], Vlastimil Babka wrote:
> > Thank you. Tested all the 6 patches in this series on Linux 5.14. This problem
> > exists in 5.13 and needs to be marked for both 5.14 and 5.13 stable releases.
> 
> I think if this problem manifests only with CONFIG_PROVE_RAW_LOCK_NESTING
> then it shouldn't be backported to stable. CONFIG_PROVE_RAW_LOCK_NESTING is
> an experimental/development option to earlier discover what will collide
> with RT lock semantics, without needing the full RT tree.
> Thus, good to fix going forward, but not necessary to stable backport.

  Acked-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
for the series. Thank you.

As for the backport I agree here with Vlastimil.

I pulled it into my RT tree for some testing and it looked good. I had
to
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3030,7 +3030,7 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
        head->func = func;
        head->next = NULL;
        local_irq_save(flags);
-       kasan_record_aux_stack(head);
+       kasan_record_aux_stack_noalloc(head);
        rdp = this_cpu_ptr(&rcu_data);
 
        /* Add the callback to our list. */

We could move kasan_record_aux_stack() before that local_irq_save() but
then call_rcu() can be called preempt-disabled section so we would have
the same problem.

The second warning came from kasan_quarantine_remove_cache(). At the end
per_cpu_remove_cache() -> qlist_free_all() will free memory with
disabled interrupts (due to that smp-function call).
Moving it to kworker would solve the problem. I don't mind keeping that
smp_function call assuming that it is all debug-code and it increases
overall latency anyway. But then could we maybe move all those objects
to a single list which freed after on_each_cpu()?

Otherwise I haven't seen any new warnings showing up with KASAN enabled.

Sebastian