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

[Jakub Kicinski]

On Wed, 18 Nov 2020 09:57:30 +0800 Yunsheng Lin wrote:
> On 2020/11/3 3:41, Jakub Kicinski wrote:
> > 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.  
> 
> Hi, Peter
> 	Any suggestion?

Let's just try lockdep_assert_in_softirq() and see if anyone complains.
People are more likely to respond to a patch than a discussion.

> >> /*
> >>  * 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().  
> 
> In that case, maybe just call lockdep_assert_in_irq() is enough?

TBH the last sentence I wrote isn't clear even to me at this point ;D

Maybe using just the macros from preempt.h - like this?

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

We know what we're doing so in_softirq() should be fine (famous last
words).