linux-kernel - Re: [PATCH RFC tip/core/rcu 38/41] rcu: Rework detection of use of RCU by offline CPUs

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Date:	Thu, 2 Feb 2012 10:31:22 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH RFC tip/core/rcu 38/41] rcu: Rework detection of use of
 RCU by offline CPUs

On Wed, Feb 01, 2012 at 10:11:06PM -0800, Josh Triplett wrote:
> On Wed, Feb 01, 2012 at 11:41:56AM -0800, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paul.mckenney@...aro.org>
> > 
> > Because newly offlined CPUs continue executing after completing the
> > CPU_DYING notifiers, they legitimately enter the scheduler and use
> > RCU while appearing to be offline.  This calls for a more sophisticated
> > approach as follows:
> > 
> > 1.	RCU marks the CPU online during the CPU_UP_PREPARE phase.
> > 
> > 2.	RCU marks the CPU offline during the CPU_DEAD phase.
> > 
> > 3.	Diagnostics regarding use of read-side RCU by offline CPUs use
> > 	RCU's accounting rather than the cpu_online_map.  (Note that
> > 	__call_rcu() still uses cpu_online_map to detect illegal
> > 	invocations within CPU_DYING notifiers.)
> > 
> > 4.	Offline CPUs are prevented from hanging the system by
> > 	force_quiescent_state(), which pays attention to cpu_online_map.
> > 	Some additional work (in a later commit) will be needed to
> > 	guarantee that force_quiescent_state() waits a full jiffy before
> > 	assuming that a CPU is offline, for example, when called from
> > 	idle entry.
> > 
> > This approach avoids the false positives encountered when attempting to
> > use more exact classification of CPU online/offline state.
> 
> Doesn't this fix need to happen *before* the earlier patches in this
> series that add splats for RCU usage while offline?  Otherwise,
> bisection can hit those splats.

If someone actually does hit the splats during a real bisection, I will
buy you the beverage of your choice.  ;-)

							Thanx, Paul

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