netdev - Re: [PATCH net v2] net: core: make napi

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Date:   Thu, 15 Apr 2021 08:46:30 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>, Lijun Pan <lijunp213@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net v2] net: core: make napi_disable more robust



On 4/15/21 1:21 AM, Jakub Kicinski wrote:
> On Wed, 14 Apr 2021 03:08:45 -0500 Lijun Pan wrote:
>> There are chances that napi_disable can be called twice by NIC driver.
>> This could generate deadlock. For example,
>> the first napi_disable will spin until NAPI_STATE_SCHED is cleared
>> by napi_complete_done, then set it again.
>> When napi_disable is called the second time, it will loop infinitely
>> because no dev->poll will be running to clear NAPI_STATE_SCHED.
>>
>> Though it is driver writer's responsibility to make sure it being
>> called only once, making napi_disable more robust does not hurt, not
>> to say it can prevent a buggy driver from crashing a system.
>> So, we check the napi state bit to make sure that if napi is already
>> disabled, we exit the call early enough to avoid spinning infinitely.
> 
> You've already been told by Eric & Dave to fix the driver instead.
> 
> Your check is _not_ correct - SCHED && NPSVC && !MISSED && !BUSY_POLL 
> can well arise without disabling the NAPI.
> 
> But regardless, a driver bug should be relatively easy to identify with
> task getting stuck in napi_disable(). We don't provide "protection" 
> for taking spin locks or ref counts twice either. Unless you can show 
> a strong use case please stop posting new versions of this patch.
> 

+222

I notice this v2 does not even mention which driver has the issue.

I suspect an out-of-tree driver.