Re: [PATCH 2/2] rcu: Fix up timeouts for forcing the quiescent state

[Paul E. McKenney]

On Mon, Sep 07, 2015 at 05:21:02PM +0200, Petr Mladek wrote:
> On Fri 2015-09-04 16:49:46, Paul E. McKenney wrote:
> > On Fri, Sep 04, 2015 at 02:11:30PM +0200, Petr Mladek wrote:
> > > The deadline to force the quiescent state (jiffies_force_qs) is currently
> > > updated only when the previous timeout passed. But the timeout used for
> > > wait_event() is always the entire original timeout. This is strange.
> > 
> > They tell me that kthreads aren't supposed to every catch signals,
> > hence the WARN_ON() in the early-exit case stray-signal case.
> 
> Yup, I have investigated this recently. All signals are really blocked
> for kthreads by default. There are few threads that use signals but
> they explicitly enable it by allow_signal().

Good!  ;-)

> > In the case where we were awakened with an explicit force-quiescent-state
> > request, we do the scan, and then wait the full time for the next scan.
> > So the point of the delay is to space out the scans, not to fit a
> > pre-determined schedule.
> > 
> > The reason we get awakened with an explicit force-quiescent-state
> > request is that a given CPU just got inundated with RCU callbacks
> > or that rcutorture wants to hammer this code path.
> > 
> > So I am not seeing this as anything in need of fixing.
> > 
> > Am I missing something subtle here?
> 
> There is the commit 88d6df612cc3c99f5 ("rcu: Prevent spurious-wakeup
> DoS attack on rcu_gp_kthread()"). It suggests that the spurious
> wakeups are possible.
> 
> I would consider this patch as a fix/clean up of this Dos attack fix.
> Huh, I forgot to mention it in the commit message.
> 
> To be honest, I personally do not know how to trigger the spurious
> wakeup in the current state of the code. I am trying to convert
> the kthread into the kthread worker API and there I got the spurious
> wakeups but this is another story.

You can do it via rcutorture, but that is not an in-production concern.

You can also do it by having all CPUs invoke call_rcu() in a tight loop.

> Thanks a lot for reviewing.

And thank you for your interest in the Linux-kernel RCU implementation!

							Thanx, Paul

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