linux-kernel - Re: net/sched: latent livelock in dev_deactivate

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Message-ID: <20170406110827.s3y6goahapq36evy@hirez.programming.kicks-ass.net>
Date:   Thu, 6 Apr 2017 13:08:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     Mike Galbraith <efault@....de>, netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: net/sched: latent livelock in dev_deactivate_many() due to
 yield() usage

On Thu, Apr 06, 2017 at 12:28:44PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 05, 2017 at 05:31:05PM -0700, Stephen Hemminger wrote:
> > On Sun, 02 Apr 2017 06:28:41 +0200
> > Mike Galbraith <efault@....de> wrote:
> > 
> > > Livelock can be triggered by setting kworkers to SCHED_FIFO, then
> > > suspend/resume.. you come back from sleepy-land with a spinning
> > > kworker.  For whatever reason, I can only do that with an enterprise
> > > like config, my standard config refuses to play, but no matter, it's
> > > "Typical broken usage".
> > > 
> > > (yield() should be rendered dead)
> > 
> > The kernel is not normally built to have kworkers run at SCHED_FIFO.
> > The user has do some action to alter the process priorities.
> > 
> > I classify this as user error. We don't support killing kworker threads
> > either.
> 
> PI can boost anybody to FIFO, all you need is to be holding a lock.

Note that this extends to rcu_read_lock(), which can cause boosting
under some cases.