Re: [PATCH net-next v2 1/2] lockdep: Introduce in_softirq lockdep assert

[Jakub Kicinski <kuba@kernel.org>]

On Mon, 23 Nov 2020 15:27:25 +0100 Peter Zijlstra wrote:
> On Sat, Nov 21, 2020 at 11:06:15AM +0800, Yunsheng Lin wrote:
> > The current semantic for napi_consume_skb() is that caller need
> > to provide non-zero budget when calling from NAPI context, and
> > breaking this semantic will cause hard to debug problem, because
> > _kfree_skb_defer() need to run in atomic context in order to push
> > the skb to the particular cpu' napi_alloc_cache atomically.
> > 
> > So add the lockdep_assert_in_softirq() to assert when the running
> > context is not in_softirq, in_softirq means softirq is serving or
> > BH is disabled. Because the softirq context can be interrupted by
> > hard IRQ or NMI context, so lockdep_assert_in_softirq() need to
> > assert about hard IRQ or NMI context too.

> Due to in_softirq() having a deprication notice (due to it being
> awefully ambiguous), could we have a nice big comment here that explains
> in detail understandable to !network people (me) why this is actually
> correct?
> 
> I'm not opposed to the thing, if that his what you need, it's fine, but
> please put on a comment that explains that in_softirq() is ambiguous and
> when you really do need it anyway.

One liner would be:

	* Acceptable for protecting per-CPU resources accessed from BH
	
We can add:

	* Much like in_softirq() - semantics are ambiguous, use carefully. *


IIUC we basically want to protect the nc array and counter here:

static inline void _kfree_skb_defer(struct sk_buff *skb)
{
	struct napi_alloc_cache *nc = this_cpu_ptr(&napi_alloc_cache);

	/* drop skb->head and call any destructors for packet */
	skb_release_all(skb);

	/* record skb to CPU local list */
	nc->skb_cache[nc->skb_count++] = skb;

#ifdef CONFIG_SLUB
	/* SLUB writes into objects when freeing */
	prefetchw(skb);
#endif

	/* flush skb_cache if it is filled */
	if (unlikely(nc->skb_count == NAPI_SKB_CACHE_SIZE)) {
		kmem_cache_free_bulk(skbuff_head_cache, NAPI_SKB_CACHE_SIZE,
				     nc->skb_cache);
		nc->skb_count = 0;
	}
}

> > +#define lockdep_assert_in_softirq()					\
> > +do {									\
> > +	WARN_ON_ONCE(__lockdep_enabled			&&		\
> > +		     (!in_softirq() || in_irq() || in_nmi()));		\
> > +} while (0)