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

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Date:   Sat, 31 Oct 2020 15:38:24 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     <davem@...emloft.net>, <linmiaohe@...wei.com>,
        <martin.varghese@...ia.com>, <pabeni@...hat.com>,
        <pshelar@....org>, <fw@...len.de>, <gnault@...hat.com>,
        <steffen.klassert@...unet.com>, <kyk.segfault@...il.com>,
        <viro@...iv.linux.org.uk>, <vladimir.oltean@....com>,
        <edumazet@...gle.com>, <saeed@...nel.org>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linuxarm@...wei.com>
Subject: Re: [PATCH net-next] net: add in_softirq() debug checking in
 napi_consume_skb()

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@...gle.com>
> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.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?

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)));                 \
+} while (0)



>  	if (!skb_unref(skb))
>  		return;
>