linux-kernel - Re: [PATCH net-next v3 1/2] lockdep: Introduce in

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Message-ID: <20201125151159.0de99e85@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 25 Nov 2020 15:11:59 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     <peterz@...radead.org>, <mingo@...hat.com>, <will@...nel.org>,
        <viro@...iv.linux.org.uk>, <kyk.segfault@...il.com>,
        <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>, <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 v3 1/2] lockdep: Introduce in_softirq lockdep
 assert

On Tue, 24 Nov 2020 18:49:28 +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, which has a ambiguous semantics due to the BH
> disabled confusion, so add a comment to emphasize that.
> 
> And the softirq context can be interrupted by hard IRQ or NMI
> context, lockdep_assert_in_softirq() need to assert about hard
> IRQ or NMI context too.
> 
> Suggested-by: Jakub Kicinski <kuba@...nel.org>
> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
> ---
> V3: add comment to emphasize the ambiguous semantics.
> ---
>  include/linux/lockdep.h | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> index f559487..8d60f46 100644
> --- a/include/linux/lockdep.h
> +++ b/include/linux/lockdep.h
> @@ -594,6 +594,13 @@ do {									\
>  		      this_cpu_read(hardirqs_enabled)));		\
>  } while (0)
>  
> +/* Much like in_softirq() - semantics are ambiguous, use carefully. */

I've added both of the comments I suggested in the reply to Peter here
and applied to net-next.

Thanks for working on this.

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