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

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Date:   Thu, 19 Nov 2020 17:19:44 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
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 2020/11/19 0:26, Jakub Kicinski wrote:
> On Wed, 18 Nov 2020 16:57:57 +0100 Peter Zijlstra wrote:
>> On Wed, Nov 18, 2020 at 07:43:48AM -0800, Jakub Kicinski wrote:
>>
>>> 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)

One thing I am not so sure about is the different irq context indicator
in preempt.h and lockdep.h, for example lockdep_assert_in_irq() uses
this_cpu_read(hardirq_context) in lockdep.h, and in_irq() uses
current_thread_info()->preempt_count in preempt.h, if they are the same
thing?

>>>
>>> We know what we're doing so in_softirq() should be fine (famous last
>>> words).  
>>
>> So that's not actually using any lockdep state. But if that's what you
>> need, I don't have any real complaints.
> 
> Great, thanks! 
> 
> The motivation was to piggy back on lockdep rather than adding a
> one-off Kconfig knob for a check in the fast path in networking.
> .
>