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

[Jakub Kicinski <kuba@kernel.org>]

On Thu, 25 Feb 2021 16:16:20 -0800 Wei Wang wrote:
> On Thu, Feb 25, 2021 at 3:00 PM Jakub Kicinski <kuba@kernel.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.

Are you concerned about it not being in RUNNING state after it's
spawned but before it's first parked?

> > > 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.

To be clear extra set_bit() only if thread is running, which if IRQ
coalescing works should be rather rare.