Re: [net] protecting interfaces from races between control and data ?

From: Luigi Rizzo <rizzo_at_iet.unipi.it>
Date: Mon, 5 Aug 2013 19:58:56 +0200
On Mon, Aug 5, 2013 at 7:49 PM, Jack Vogel <jfvogel_at_gmail.com> wrote:

> Sigh, this ends up being ugly I'm afraid. I need some time to look at code
> and think about it.
>
>
actually the intel drivers seem in decent shape,
especially if we reuse IFF_DRV_RUNNING as the reset flag
and the core+queue lock in the control path.

cheers
luigi



> Jack
>
>
>
> On Mon, Aug 5, 2013 at 10:36 AM, Luigi Rizzo <rizzo_at_iet.unipi.it> wrote:
>
>> On Mon, Aug 5, 2013 at 7:17 PM, Adrian Chadd <adrian_at_freebsd.org> wrote:
>>
>> > I'm travelling back to San Jose today; poke me tomorrow and I'll brain
>> > dump what I did in ath(4) and the lessons learnt.
>> >
>> > The TL;DR version - you don't want to grab an extra lock in the
>> > read/write paths as that slows things down. Reuse the same per-queue
>> > TX/RX lock and have:
>> >
>> > * a reset flag that is set when something is resetting; that says to
>> > the queue "don't bother processing anything, just dive out";
>> > * 'i am doing Tx / Rx' flags per queue that is set at the start of
>> > TX/RX servicing and finishes at the end; that way the reset code knows
>> > if there's something pending;
>> > * have the reset path grab each lock, set the 'reset' flag on each,
>> > then walk each queue again and make sure they're all marked as 'not
>> > doing TX/RX'. At that point the reset can occur, then the flag cna be
>> > cleared, then TX/RX can resume.
>> >
>>
>> so this is slightly different from what Bryan suggested (and you endorsed)
>> before, as in that case there was a single 'reset' flag IFF_DRV_RUNNING
>> protected by the 'core' lock, then a nested round on all tx and rx locks
>> to make sure that all customers have seen it.
>> In both cases the tx and rx paths only need the per-queue lock.
>>
>> As i see it, having a per-queue reset flag removes the need for nesting
>> core + queue locks, but since this is only in the control path perhaps
>> it is not a big deal (and is better to have a single place to look at to
>> tell whether or not we should bail out).
>>
>> cheers
>> luigi
>>
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>
>


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Received on Mon Aug 05 2013 - 15:58:59 UTC

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