Messages in this thread | | | Date | Wed, 18 Nov 2020 07:43:48 -0800 | From | Jakub Kicinski <> | Subject | Re: [PATCH net-next] net: add in_softirq() debug checking in napi_consume_skb() |
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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@google.com> > >>>> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com> > >>> > >>>> 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).
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