linux-kernel - Re: [PATCH RT] net: Have __napi_schedule

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Date:   Wed, 7 Dec 2016 10:28:34 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Clark Williams <williams@...hat.com>,
        "Luis Claudio R. Goncalves" <lclaudio@...g.org>,
        John Kacur <jkacur@...hat.com>
Subject: Re: [PATCH RT] net: Have __napi_schedule_irqoff() disable
 interrupts on RT

On Wed, 7 Dec 2016 07:53:26 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Wed, 7 Dec 2016 10:10:40 +0100
> Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> 
> > On 2016-12-06 17:50:30 [-0500], Steven Rostedt wrote:  
> > > A customer hit a crash where the napi sd->poll_list became corrupted.
> > > The customer had the bnx2x driver, which does a
> > > __napi_schedule_irqoff() in its interrupt handler. Unfortunately, when
> > > running with CONFIG_PREEMPT_RT_FULL, this interrupt handler is run as a
> > > thread and is preemptable. The call to ____napi_schedule() must be done
> > > with interrupts disabled to protect the per cpu softnet_data's
> > > poll_list, which is protected by disabling interrupts (disabling
> > > preemption is enough when all interrupts are threaded and
> > > local_bh_disable() can't preempt).
> > > 
> > > As bnx2x isn't the only driver that does this, the safest thing to do
> > > is to make __napi_schedule_irqoff() call __napi_schedule() instead when
> > > CONFIG_PREEMPT_RT_FULL is enabled, which will call local_irq_save()
> > > before calling ____napi_schedule().    
> > 
> > It would work RT wise. But don't have the same problem if you boot the
> > kernel with threadirqs ?
> >   
> 
> I thought the same at first, but looking into the code for forced
> threaded interrupts, I saw this:
> 
>         local_bh_disable();
>         ret = action->thread_fn(action->irq, action->dev_id);
>         irq_finalize_oneshot(desc, action);
>         local_bh_enable();
> 
> Where without CONFIG_PREEMPT_RT_FULL, local_bh_disable() also disables
> preemption. Then all the handlers still can not be preempted by another
> handler. So it appears to be safe as well.
> 

Actually, I was hoping that the change log explained this. Should I
reword it better?

"poll_list, which is protected by disabling interrupts (disabling
 preemption is enough when all interrupts are threaded and
 local_bh_disable() can't preempt)."


-- Steve