linux-kernel - Re: [PATCH tip/core/rcu 11/15] rcu: Avoid spurious RCU CPU stall warnings

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Message-ID: <1347028929.8178.8.camel@gandalf.local.home>
Date:	Fri, 07 Sep 2012 10:42:09 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/rcu 11/15] rcu: Avoid spurious RCU CPU stall
 warnings

On Fri, 2012-09-07 at 09:00 +0200, Peter Zijlstra wrote:
> On Thu, 2012-09-06 at 15:22 -0700, Paul E. McKenney wrote:
> > Ah!
> > 
> > It is perfectly legal to avoid -starting- an RCU grace period for a
> > minute, or even longer.  If RCU has nothing to do, in other words, if no
> > one registers any RCU callbacks, then RCU need not start a grace period.
> > 
> > Of course, this would mean that it would eventually be a full minute
> > since the last start of a grace period.  This is not a problem, after
> > all, Linux went through a full ten years before experiencing its first
> > grace period.
> > 
> > But the stall-warning code just checked how long it had been since
> > the last start of a grace period, failing to note that this grace
> > period had long since completed.  So it splatted out a warning.
> > This warning was spurious in the sense that there was no bug aside
> > from the missing check that the grace period was still in progress.
> > 
> > And this commit fixes that bug in RCU. 
> 
> OK, that makes sense.. it just looks like both Steve and me got confused
> by the initial changelog. 

Right, I think the change log needs to be fixed ;-)

-- Steve


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