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

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Message-ID: <20150907152102.GC9577@pathway.suse.cz>
Date:	Mon, 7 Sep 2015 17:21:02 +0200
From:	Petr Mladek <pmladek@...e.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Jiri Kosina <jkosina@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] rcu: Fix up timeouts for forcing the quiescent state

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().


> 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.

Thanks a lot for reviewing.

Best Regards,
Petr
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