Re: call_rcu from trace_preempt

Paul E. McKenney
2015-06-16T14:16:46Z

On Tue, Jun 16, 2015 at 02:38:53PM +0200, Daniel Wagner wrote:
> On 06/16/2015 02:27 PM, Paul E. McKenney wrote:
> > On Mon, Jun 15, 2015 at 10:45:05PM -0700, Alexei Starovoitov wrote:
> >> On 6/15/15 7:14 PM, Paul E. McKenney wrote:
> >>>
> >>> Why do you believe that it is better to fix it within call_rcu()?
> >>
> >> found it:
> >> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> >> index 8cf7304b2867..a3be09d482ae 100644
> >> --- a/kernel/rcu/tree.c
> >> +++ b/kernel/rcu/tree.c
> >> @@ -935,9 +935,9 @@ bool notrace rcu_is_watching(void)
> >> {
> >> bool ret;
> >>
> >> - preempt_disable();
> >> + preempt_disable_notrace();
> >> ret = __rcu_is_watching();
> >> - preempt_enable();
> >> + preempt_enable_notrace();
> >> return ret;
> >> }
> >>
> >> the rcu_is_watching() and __rcu_is_watching() are already marked
> >> notrace, so imo it's a good 'fix'.
> >> What was happening is that the above preempt_enable was triggering
> >> recursive call_rcu that was indeed messing 'rdp' that was
> >> prepared by __call_rcu and before __call_rcu_core could use that.
> >
> >> btw, also noticed that local_irq_save done by note_gp_changes
> >> is partially redundant. In __call_rcu_core path the irqs are
> >> already disabled.
> >
> > But you said earlier that nothing happened when interrupts were
> > disabled. And interrupts are disabled across the call to
> > rcu_is_watching() in __call_rcu_core(). So why did those calls
> > to preempt_disable() and preempt_enable() cause trouble?
>
> Just for the record: Using a thread for freeing the memory is curing the
> problem without the need to modify rcu_is_watching.

I must confess to liking this approach better than guaranteeing full-up
reentrancy in call_rcu() and kfree_rcu(). ;-)

Thanx, Paul

> Here is the hack:
>
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 83c209d..8d73be3 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -13,6 +13,8 @@
> #include <linux/jhash.h>
> #include <linux/filter.h>
> #include <linux/vmalloc.h>
> +#include <linux/kthread.h>
> +#include <linux/spinlock.h>
>
> struct bpf_htab {
> struct bpf_map map;
> @@ -27,10 +29,14 @@ struct bpf_htab {
> struct htab_elem {
> struct hlist_node hash_node;
> struct rcu_head rcu;
> + struct list_head list;
> u32 hash;
> char key[0] __aligned(8);
> };
>
> +static LIST_HEAD(elem_freelist);
> +static DEFINE_SPINLOCK(elem_freelist_lock);
> +
> /* Called from syscall */
> static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
> {
> @@ -228,6 +234,7 @@ static int htab_map_update_elem(struct bpf_map *map,
> void *key, void *value,
> memcpy(l_new->key + round_up(key_size, 8), value, map->value_size);
>
> l_new->hash = htab_map_hash(l_new->key, key_size);
> + INIT_LIST_HEAD(&l_new->list);
>
> /* bpf_map_update_elem() can be called in_irq() */
> spin_lock_irqsave(&htab->lock, flags);
> @@ -300,11 +307,17 @@ static int htab_map_delete_elem(struct bpf_map
> *map, void *key)
> if (l) {
> hlist_del_rcu(&l->hash_node);
> htab->count--;
> - kfree_rcu(l, rcu);
> + /* kfree_rcu(l, rcu); */
> ret = 0;
> }
>
> spin_unlock_irqrestore(&htab->lock, flags);
> +
> + if (l) {
> + spin_lock_irqsave(&elem_freelist_lock, flags);
> + list_add(&l->list, &elem_freelist);
> + spin_unlock_irqrestore(&elem_freelist_lock, flags);
> + }
> return ret;
> }
>
> @@ -359,9 +372,31 @@ static struct bpf_map_type_list htab_type
> __read_mostly = {
> .type = BPF_MAP_TYPE_HASH,
> };
>
> +static int free_thread(void *arg)
> +{
> + unsigned long flags;
> + struct htab_elem *l;
> +
> + while (!kthread_should_stop()) {
> + spin_lock_irqsave(&elem_freelist_lock, flags);
> + while (!list_empty(&elem_freelist)) {
> + l = list_entry(elem_freelist.next,
> + struct htab_elem, list);
> + list_del(&l->list);
> + kfree(l);
> + }
> + spin_unlock_irqrestore(&elem_freelist_lock, flags);
> + }
> +
> + return 0;
> +}
> +
> static int __init register_htab_map(void)
> {
> bpf_register_map_type(&htab_type);
> +
> + kthread_run(free_thread, NULL, "free_thread");
> +
> return 0;
> }
> late_initcall(register_htab_map);
>