netdev - Re: [PATCH] net: Prevent multiple NAPI instances co-existing in the list

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Message-ID: <CA+U0gVgZr-Mhzest1O_vtCWMwuaQmP30DYb8CLXDG8iTobObYQ@mail.gmail.com>
Date:	Fri, 9 Jan 2015 10:24:18 +0800
From:	Dennis Chen <kernel.org.gnu@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	netdev <netdev@...r.kernel.org>, davem@...emloft.net,
	eric.dumazet@...il.com
Subject: Re: [PATCH] net: Prevent multiple NAPI instances co-existing in the list

On Thu, Jan 8, 2015 at 7:15 PM, Herbert Xu <herbert@...dor.apana.org.au> wrote:
> Dennis Chen <kernel.org.gnu@...il.com> wrote:
>> Some drivers may clear the NAPI_STATE_SCHED bit upon the state of the
>> NAPI instance after exhaust the budget in the poll function, which
>> will open a window for next device interrupt handler to insert a same
>> instance to the list after calling list_add_tail(&n->poll_list,
>> repoll) if we don't set this bit.
>>
>> Signed-off-by: Dennis Chen <kernel.org.gnu@...il.com>
>
> Which driver is doing this?
>
> Cheers,
> --
> Email: Herbert Xu <herbert@...dor.apana.org.au>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Hi Herbert, please see this code piece in napi_poll:

     /* Some drivers may have called napi_schedule
     * prior to exhausting their budget.
     */
    if (unlikely(!list_empty(&n->poll_list))) {
        pr_warn_once("%s: Budget exhausted after napi rescheduled\n",
                 n->dev ? n->dev->name : "backlog");
        goto out_unlock;
    }

Here "Some drivers" may have called  napi_schedule to make
n->poll_list is not empty, does that mean "Some drivers" will clear
NAPI_STATE_SCHED bit, otherwise the napi_schedule() will do nothing,
does that make sense for you question? ;-)

-- 
Den
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