netdev - Re: [PATCH net] net: fix race between napi kthread mode and busy poll

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Date:   Thu, 25 Feb 2021 16:16:20 -0800
From:   Wei Wang <weiwan@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Alexander Duyck <alexanderduyck@...com>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Martin Zaharinov <micron10@...il.com>
Subject: Re: [PATCH net] net: fix race between napi kthread mode and busy poll

On Thu, Feb 25, 2021 at 3:00 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Thu, 25 Feb 2021 10:29:47 -0800 Wei Wang wrote:
> > Hmm... I don't think the above patch would work. Consider a situation that:
> > 1. At first, the kthread is in sleep mode.
> > 2. Then someone calls napi_schedule() to schedule work on this napi.
> > So ____napi_schedule() is called. But at this moment, the kthread is
> > not yet in RUNNING state. So this function does not set SCHED_THREAD
> > bit.
> > 3. Then wake_up_process() is called to wake up the thread.
> > 4. Then napi_threaded_poll() calls napi_thread_wait().
>
> But how is the task not in running state outside of napi_thread_wait()?
>
> My scheduler knowledge is rudimentary, but AFAIU off CPU tasks which
> were not put to sleep are still in RUNNING state, so unless we set
> INTERRUPTIBLE the task will be running, even if it's stuck in cond_resched().
>

I think the thread is only in RUNNING state after wake_up_process() is
called on the thread in ____napi_schedule(). Before that, it should be
in INTERRUPTIBLE state. napi_thread_wait() explicitly calls
set_current_state(TASK_INTERRUPTIBLE) when it finishes 1 round of
polling.

> > woken is false
> > and SCHED_THREAD bit is not set. So the kthread will go to sleep again
> > (in INTERRUPTIBLE mode) when schedule() is called, and waits to be
> > woken up by the next napi_schedule().
> > That will introduce arbitrary delay for the napi->poll() to be called.
> > Isn't it? Please enlighten me if I did not understand it correctly.
>
> Probably just me not understanding the scheduler :)
>
> > I personally prefer to directly set SCHED_THREAD bit in ____napi_schedule().
> > Or stick with SCHED_BUSY_POLL solution and replace kthread_run() with
> > kthread_create().
>
> Well, I'm fine with that too, no point arguing further if I'm not
> convincing anyone. But we need a fix which fixes the issue completely,
> not just one of three incarnations.

Alexander and Eric,
Do you guys have preference on which approach to take?
If we keep the current SCHED_BUSY_POLL patch, I think we need to
change kthread_run() to kthread_create() to address the warning Martin
reported.
Or if we choose to set SCHED_THREADED, we could keep kthread_run().
But there is 1 extra set_bit() operation.

Thanks.
Wei