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Date:	Tue, 16 Jun 2015 07:16:26 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Daniel Wagner <wagi@...om.org>
Cc:	Alexei Starovoitov <ast@...mgrid.com>,
	Daniel Wagner <daniel.wagner@...-carit.de>,
	LKML <linux-kernel@...r.kernel.org>, rostedt@...dmis.org
Subject: Re: call_rcu from trace_preempt

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);
> 

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