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

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Message-ID: <1347001230.18408.72.camel@twins>
Date:	Fri, 07 Sep 2012 09:00:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Steven Rostedt <rostedt@...dmis.org>, 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 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. 
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