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From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Josh Triplett <josh@joshtriplett.org>
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu,
	laijs@cn.fujitsu.com, dipankar@in.ibm.com,
	akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca,
	niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org,
	rostedt@goodmis.org, Valdis.Kletnieks@vt.edu,
	dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com,
	fweisbec@gmail.com, sbw@mit.edu, patches@linaro.org
Subject: Re: [PATCH tip/core/rcu 16/23] rcu: Prevent initialization-time quiescent-state race
Date: Wed, 5 Sep 2012 12:49:18 -0700	[thread overview]
Message-ID: <20120905194918.GQ3308@linux.vnet.ibm.com> (raw)
In-Reply-To: <20120905185534.GA4976@jtriplet-mobl1>

On Wed, Sep 05, 2012 at 11:55:34AM -0700, Josh Triplett wrote:
> On Wed, Sep 05, 2012 at 11:19:20AM -0700, Paul E. McKenney wrote:
> > On Mon, Sep 03, 2012 at 02:37:42AM -0700, Josh Triplett wrote:
> > > On Thu, Aug 30, 2012 at 11:18:31AM -0700, Paul E. McKenney wrote:
> > > > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > > > 
> > > > Now the the grace-period initialization procedure is preemptible, it is
> > > > subject to the following race on systems whose rcu_node tree contains
> > > > more than one node:
> > > > 
> > > > 1.	CPU 31 starts initializing the grace period, including the
> > > > 	first leaf rcu_node structures, and is then preempted.
> > > > 
> > > > 2.	CPU 0 refers to the first leaf rcu_node structure, and notes
> > > > 	that a new grace period has started.  It passes through a
> > > > 	quiescent state shortly thereafter, and informs the RCU core
> > > > 	of this rite of passage.
> > > > 
> > > > 3.	CPU 0 enters an RCU read-side critical section, acquiring
> > > > 	a pointer to an RCU-protected data item.
> > > > 
> > > > 4.	CPU 31 removes the data item referenced by CPU 0 from the
> > > > 	data structure, and registers an RCU callback in order to
> > > > 	free it.
> > > > 
> > > > 5.	CPU 31 resumes initializing the grace period, including its
> > > > 	own rcu_node structure.  In invokes rcu_start_gp_per_cpu(),
> > > > 	which advances all callbacks, including the one registered
> > > > 	in #4 above, to be handled by the current grace period.
> > > > 
> > > > 6.	The remaining CPUs pass through quiescent states and inform
> > > > 	the RCU core, but CPU 0 remains in its RCU read-side critical
> > > > 	section, still referencing the now-removed data item.
> > > > 
> > > > 7.	The grace period completes and all the callbacks are invoked,
> > > > 	including the one that frees the data item that CPU 0 is still
> > > > 	referencing.  Oops!!!
> > > > 
> > > > This commit therefore moves the callback handling to precede initialization
> > > > of any of the rcu_node structures, thus avoiding this race.
> > > 
> > > I don't think it makes sense to introduce and subsequently fix a race in
> > > the same patch series. :)
> > > 
> > > Could you squash this patch into the one moving grace-period
> > > initialization into a kthread?
> > 
> > I tried that, and got a surprisingly large set of conflicts.  Ah, OK,
> > the problem is that breaking up rcu_gp_kthread() into subfunctions
> > did enough code motion to defeat straightforward rebasing.  Is there
> > some way to tell "git rebase" about such code motion, or would this
> > need to be carried out carefully by hand?
> 
> To the extent rebase knows how to handle that, I think it does so
> automatically as part of merge attempts.  Fortunately, in this case, the
> change consists of moving two lines of code and their attached comment,
> which seems easy enough to change in the original code; you'll then get
> a conflict on the commit that moves the newly fixed code (easily
> resolved by moving the change to the new code), and conflicts on any
> changes next to the change in the new code (hopefully handled by
> three-way merge, and if not then easily fixed by keeping the new lines).

Good point, perhaps if I do the code movement manually and use multiple
rebases it will go more easily.

							Thanx, Paul


  reply	other threads:[~2012-09-05 19:50 UTC|newest]

Thread overview: 85+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-08-30 18:18 [PATCH tip/core/rcu 0/23] Improvements to RT response on big systems and expedited functions Paul E. McKenney
2012-08-30 18:18 ` [PATCH tip/core/rcu 01/23] rcu: Move RCU grace-period initialization into a kthread Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 02/23] rcu: Allow RCU grace-period initialization to be preempted Paul E. McKenney
2012-09-02  1:09     ` Josh Triplett
2012-09-05  1:22       ` Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 03/23] rcu: Move RCU grace-period cleanup into kthread Paul E. McKenney
2012-09-02  1:22     ` Josh Triplett
2012-09-06 13:34     ` Peter Zijlstra
2012-09-06 17:29       ` Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 04/23] rcu: Allow RCU grace-period cleanup to be preempted Paul E. McKenney
2012-09-02  1:36     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 05/23] rcu: Prevent offline CPUs from executing RCU core code Paul E. McKenney
2012-09-02  1:45     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 06/23] rcu: Break up rcu_gp_kthread() into subfunctions Paul E. McKenney
2012-09-02  2:11     ` Josh Triplett
2012-09-06 13:39     ` Peter Zijlstra
2012-09-06 17:32       ` Paul E. McKenney
2012-09-06 18:49         ` Josh Triplett
2012-09-06 19:09           ` Peter Zijlstra
2012-09-06 20:30             ` Paul E. McKenney
2012-09-06 20:30           ` Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 07/23] rcu: Provide OOM handler to motivate lazy RCU callbacks Paul E. McKenney
2012-09-02  2:13     ` Josh Triplett
2012-09-03  9:08     ` Lai Jiangshan
2012-09-05 17:45       ` Paul E. McKenney
2012-09-06 13:46     ` Peter Zijlstra
2012-09-06 13:52       ` Steven Rostedt
2012-09-06 17:41         ` Paul E. McKenney
2012-09-06 17:46           ` Peter Zijlstra
2012-09-06 20:32             ` Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 08/23] rcu: Segregate rcu_state fields to improve cache locality Paul E. McKenney
2012-09-02  2:51     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 09/23] rcu: Move quiescent-state forcing into kthread Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 10/23] rcu: Allow RCU quiescent-state forcing to be preempted Paul E. McKenney
2012-09-02  5:23     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 11/23] rcu: Adjust debugfs tracing for kthread-based quiescent-state forcing Paul E. McKenney
2012-09-02  6:05     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 12/23] rcu: Prevent force_quiescent_state() memory contention Paul E. McKenney
2012-09-02 10:47     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 13/23] rcu: Control grace-period duration from sysfs Paul E. McKenney
2012-09-03  9:30     ` Josh Triplett
2012-09-03  9:31       ` Josh Triplett
2012-09-06 14:15     ` Peter Zijlstra
2012-09-06 17:53       ` Paul E. McKenney
2012-09-06 18:28         ` Peter Zijlstra
2012-09-06 20:37           ` Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 14/23] rcu: Remove now-unused rcu_state fields Paul E. McKenney
2012-09-03  9:31     ` Josh Triplett
2012-09-06 14:17     ` Peter Zijlstra
2012-09-06 18:02       ` Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 15/23] rcu: Make rcutree module parameters visible in sysfs Paul E. McKenney
2012-09-03  9:32     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 16/23] rcu: Prevent initialization-time quiescent-state race Paul E. McKenney
2012-09-03  9:37     ` Josh Triplett
2012-09-05 18:19       ` Paul E. McKenney
2012-09-05 18:55         ` Josh Triplett
2012-09-05 19:49           ` Paul E. McKenney [this message]
2012-09-06 14:21         ` Peter Zijlstra
2012-09-06 16:18           ` Paul E. McKenney
2012-09-06 16:22             ` Peter Zijlstra
2012-08-30 18:18   ` [PATCH tip/core/rcu 17/23] rcu: Fix day-zero grace-period initialization/cleanup race Paul E. McKenney
2012-09-03  9:39     ` Josh Triplett
2012-09-06 14:24     ` Peter Zijlstra
2012-09-06 18:06       ` Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 18/23] rcu: Add random PROVE_RCU_DELAY to grace-period initialization Paul E. McKenney
2012-09-03  9:41     ` Josh Triplett
2012-09-06 14:27     ` Peter Zijlstra
2012-09-06 18:25       ` Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 19/23] rcu: Adjust for unconditional ->completed assignment Paul E. McKenney
2012-09-03  9:42     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 20/23] rcu: Remove callback acceleration from grace-period initialization Paul E. McKenney
2012-09-03  9:42     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 21/23] rcu: Eliminate signed overflow in synchronize_rcu_expedited() Paul E. McKenney
2012-09-03  9:43     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 22/23] rcu: Reduce synchronize_rcu_expedited() latency Paul E. McKenney
2012-09-03  9:46     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 23/23] rcu: Simplify quiescent-state detection Paul E. McKenney
2012-09-03  9:56     ` Josh Triplett
2012-09-06 14:36     ` Peter Zijlstra
2012-09-06 20:01       ` Paul E. McKenney
2012-09-06 21:18         ` Mathieu Desnoyers
2012-09-06 21:31           ` Paul E. McKenney
2012-09-02  1:04   ` [PATCH tip/core/rcu 01/23] rcu: Move RCU grace-period initialization into a kthread Josh Triplett
2012-09-06 13:32   ` Peter Zijlstra
2012-09-06 17:00     ` Paul E. McKenney

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