Re: [PATCH net-next] net: add in_softirq() debug checking in napi_consume_skb()

[Jakub Kicinski]

On Mon, 2 Nov 2020 11:14:32 +0800 Yunsheng Lin wrote:
> On 2020/11/1 6:38, Jakub Kicinski wrote:
> > On Thu, 29 Oct 2020 19:34:48 +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 a in_softirq() debug checking in napi_consume_skb() to catch
> >> this kind of error.
> >>
> >> Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> >> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>  
> >   
> >> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> >> index 1ba8f01..1834007 100644
> >> --- a/net/core/skbuff.c
> >> +++ b/net/core/skbuff.c
> >> @@ -897,6 +897,10 @@ void napi_consume_skb(struct sk_buff *skb, int budget)
> >>  		return;
> >>  	}
> >>  
> >> +	DEBUG_NET_WARN(!in_softirq(),
> >> +		       "%s is called with non-zero budget outside softirq context.\n",
> >> +		       __func__);  
> > 
> > Can't we use lockdep instead of defining our own knobs?  
> 
> From the first look, using the below seems better than defining our
> own knobs, because there is similar lockdep_assert_in_irq() checking
> already and lockdep_assert_in_*() is NULL-OP when CONFIG_PROVE_LOCKING
> is not defined.
> 
> > 
> > Like this maybe?
> > 
> > diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> > index f5594879175a..5253a167d00c 100644
> > --- a/include/linux/lockdep.h
> > +++ b/include/linux/lockdep.h
> > @@ -594,6 +594,14 @@ do {                                                                       \
> >                       this_cpu_read(hardirqs_enabled)));                \
> >  } while (0)
> >  
> > +#define lockdep_assert_in_softirq()                                    \
> > +do {                                                                   \
> > +       WARN_ON_ONCE(__lockdep_enabled                  &&              \
> > +                    (softirq_count() == 0              ||              \
> > +                     this_cpu_read(hardirq_context)));                 \  
> 
> Using in_softirq() seems more obvious then using softirq_count()?
> And there is below comment above avoiding the using of in_softirq(), maybe
> that is why you use softirq_count() directly here?
> "softirq_count() == 0" still mean we are not in the SoftIRQ context and
> BH is not disabled. right? Perhap lockdep_assert_in_softirq_or_bh_disabled()
> is more obvious?

Let's add Peter to the recipients to get his opinion.

We have a per-cpu resource which is also accessed from BH (see
_kfree_skb_defer()).

We're trying to come up with the correct lockdep incantation for it.

> /*
>  * Are we doing bottom half or hardware interrupt processing?
>  *
>  * in_irq()       - We're in (hard) IRQ context
>  * in_softirq()   - We have BH disabled, or are processing softirqs
>  * in_interrupt() - We're in NMI,IRQ,SoftIRQ context or have BH disabled
>  * in_serving_softirq() - We're in softirq context
>  * in_nmi()       - We're in NMI context
>  * in_task()	  - We're in task context
>  *
>  * Note: due to the BH disabled confusion: in_softirq(),in_interrupt() really
>  *       should not be used in new code.
>  */
> 
> 
> Also, is there any particular reason we do the "this_cpu_read(hardirq_context)"
> checking?

Accessing BH resources from a hard IRQ context would be a bug, we may
have interrupted a BH, so AFAIU softirq_count() != 0, but we can nest
calls to _kfree_skb_defer().