Message-ID: <20070910184240.GI3819@linux.vnet.ibm.com>
Date:	Mon, 10 Sep 2007 11:42:40 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	linux-rt-users@...r.kernel.org, mingo@...e.hu,
	akpm@...ux-foundation.org, dipankar@...ibm.com,
	josht@...ux.vnet.ibm.com, tytso@...ibm.com, dvhltc@...ibm.com,
	tglx@...utronix.de, a.p.zijlstra@...llo.nl, bunk@...nel.org,
	ego@...ibm.com, oleg@...sign.ru, srostedt@...hat.com
Subject: [PATCH RFC 9/9] RCU: preemptible documentation and comment cleanups

Work in progress, not for inclusion.

This patch updates the RCU documentation to reflect preemptible RCU as
well as recent publications.  Fix an incorrect comment in the code. 
Change the name ORDERED_WRT_IRQ() to ACCESS_ONCE() to better describe
its function.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---

 Documentation/RCU/RTFP.txt    |  234 ++++++++++++++++++++++++++++++++++++++++--
 Documentation/RCU/rcu.txt     |   20 +++
 Documentation/RCU/torture.txt |   44 ++++++-
 kernel/Kconfig.preempt        |    1 
 kernel/rcupreempt.c           |   24 ++--
 5 files changed, 292 insertions(+), 31 deletions(-)

diff -urpNa -X dontdiff linux-2.6.22-h-boostsleep/Documentation/RCU/rcu.txt linux-2.6.22-i-cleanups/Documentation/RCU/rcu.txt
--- linux-2.6.22-h-boostsleep/Documentation/RCU/rcu.txt	2007-07-08 16:32:17.000000000 -0700
+++ linux-2.6.22-i-cleanups/Documentation/RCU/rcu.txt	2007-09-08 16:54:17.000000000 -0700
@@ -36,6 +36,14 @@ o	How can the updater tell when a grace 
 	executed in user mode, or executed in the idle loop, we can
 	safely free up that item.
 
+	Preemptible variants of RCU (CONFIG_PREEMPT_RCU) get the
+	same effect, but require that the readers manipulate CPU-local
+	counters.  These counters allow limited types of blocking
+	within RCU read-side critical sections.  SRCU also uses
+	CPU-local counters, and permits general blocking within
+	RCU read-side critical sections.  These two variants of
+	RCU detect grace periods by sampling these counters.
+
 o	If I am running on a uniprocessor kernel, which can only do one
 	thing at a time, why should I wait for a grace period?
 
@@ -46,7 +54,10 @@ o	How can I see where RCU is currently u
 	Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
 	"rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh",
 	"srcu_read_lock", "srcu_read_unlock", "synchronize_rcu",
-	"synchronize_net", and "synchronize_srcu".
+	"synchronize_net", "synchronize_srcu", and the other RCU
+	primitives.  Or grab one of the cscope databases from:
+
+	http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html
 
 o	What guidelines should I follow when writing code that uses RCU?
 
@@ -67,7 +78,12 @@ o	I hear that RCU is patented?  What is 
 
 o	I hear that RCU needs work in order to support realtime kernels?
 
-	Yes, work in progress.
+	This work is largely completed.  Realtime-friendly RCU can be
+	enabled via the CONFIG_PREEMPT_RCU kernel configuration parameter.
+	In addition, the CONFIG_PREEMPT_RCU_BOOST kernel configuration
+	parameter enables priority boosting of preempted RCU read-side
+	critical sections, though this is only needed if you have
+	CPU-bound realtime threads.
 
 o	Where can I find more information on RCU?
 
diff -urpNa -X dontdiff linux-2.6.22-h-boostsleep/Documentation/RCU/RTFP.txt linux-2.6.22-i-cleanups/Documentation/RCU/RTFP.txt
--- linux-2.6.22-h-boostsleep/Documentation/RCU/RTFP.txt	2007-07-08 16:32:17.000000000 -0700
+++ linux-2.6.22-i-cleanups/Documentation/RCU/RTFP.txt	2007-09-08 16:41:31.000000000 -0700
@@ -9,8 +9,8 @@ The first thing resembling RCU was publi
 [Kung80] recommended use of a garbage collector to defer destruction
 of nodes in a parallel binary search tree in order to simplify its
 implementation.  This works well in environments that have garbage
-collectors, but current production garbage collectors incur significant
-read-side overhead.
+collectors, but most production garbage collectors incur significant
+overhead.
 
 In 1982, Manber and Ladner [Manber82,Manber84] recommended deferring
 destruction until all threads running at that time have terminated, again
@@ -99,16 +99,25 @@ locking, reduces contention, reduces mem
 parallelizes pipeline stalls and memory latency for writers.  However,
 these techniques still impose significant read-side overhead in the
 form of memory barriers.  Researchers at Sun worked along similar lines
-in the same timeframe [HerlihyLM02,HerlihyLMS03].  These techniques
-can be thought of as inside-out reference counts, where the count is
-represented by the number of hazard pointers referencing a given data
-structure (rather than the more conventional counter field within the
-data structure itself).
+in the same timeframe [HerlihyLM02].  These techniques can be thought
+of as inside-out reference counts, where the count is represented by the
+number of hazard pointers referencing a given data structure (rather than
+the more conventional counter field within the data structure itself).
+
+By the same token, RCU can be thought of as a "bulk reference count",
+where some form of reference counter covers all reference by a given CPU
+or thread during a set timeframe.  This timeframe is related to, but
+not necessarily exactly the same as, an RCU grace period.  In classic
+RCU, the reference counter is the per-CPU bit in the "bitmask" field,
+and each such bit covers all references that might have been made by
+the corresponding CPU during the prior grace period.  Of course, RCU
+can be thought of in other terms as well.
 
 In 2003, the K42 group described how RCU could be used to create
-hot-pluggable implementations of operating-system functions.  Later that
-year saw a paper describing an RCU implementation of System V IPC
-[Arcangeli03], and an introduction to RCU in Linux Journal [McKenney03a].
+hot-pluggable implementations of operating-system functions [Appavoo03a].
+Later that year saw a paper describing an RCU implementation of System
+V IPC [Arcangeli03], and an introduction to RCU in Linux Journal
+[McKenney03a].
 
 2004 has seen a Linux-Journal article on use of RCU in dcache
 [McKenney04a], a performance comparison of locking to RCU on several
@@ -117,10 +126,27 @@ number of operating-system kernels [Paul
 describing how to make RCU safe for soft-realtime applications [Sarma04c],
 and a paper describing SELinux performance with RCU [JamesMorris04b].
 
-2005 has seen further adaptation of RCU to realtime use, permitting
+2005 brought further adaptation of RCU to realtime use, permitting
 preemption of RCU realtime critical sections [PaulMcKenney05a,
 PaulMcKenney05b].
 
+2006 saw the first best-paper award for an RCU paper [ThomasEHart2006a],
+as well as further work on efficient implementations of preemptible
+RCU [PaulEMcKenney2006b], but priority-boosting of RCU read-side critical
+sections proved elusive.  An RCU implementation permitting general
+blocking in read-side critical sections appeared [PaulEMcKenney2006c],
+Robert Olsson described an RCU-protected trie-hash combination
+[RobertOlsson2006a].
+
+In 2007, the RCU priority-boosting problem finally was solved
+[PaulEMcKenney2007BoostRCU], and an RCU paper was first accepted into
+an academic journal [ThomasEHart2007a].  An LWN article on the use of
+Promela and spin to validate parallel algorithms [PaulEMcKenney2007QRCUspin]
+also described Oleg Nesterov's QRCU, the first RCU implementation that
+can boast deep sub-microsecond grace periods (in absence of readers,
+and read-side overhead is roughly that of a global reference count).
+
+
 Bibtex Entries
 
 @article{Kung80
@@ -203,6 +229,41 @@ Bibtex Entries
 ,Address="New Orleans, LA"
 }
 
+@...ference{Pu95a,
+Author = "Calton Pu and Tito Autrey and Andrew Black and Charles Consel and
+Crispin Cowan and Jon Inouye and Lakshmi Kethana and Jonathan Walpole and
+Ke Zhang",
+Title = "Optimistic Incremental Specialization: Streamlining a Commercial
+Operating System",
+Booktitle = "15\textsuperscript{th} ACM Symposium on
+Operating Systems Principles (SOSP'95)",
+address = "Copper Mountain, CO",
+month="December",
+year="1995",
+pages="314-321",
+annotation="
+	Uses a replugger, but with a flag to signal when people are
+	using the resource at hand.  Only one reader at a time.
+"
+}
+
+@...ference{Cowan96a,
+Author = "Crispin Cowan and Tito Autrey and Charles Krasic and
+Calton Pu and Jonathan Walpole",
+Title = "Fast Concurrent Dynamic Linking for an Adaptive Operating System",
+Booktitle = "International Conference on Configurable Distributed Systems
+(ICCDS'96)",
+address = "Annapolis, MD",
+month="May",
+year="1996",
+pages="108",
+isbn="0-8186-7395-8",
+annotation="
+	Uses a replugger, but with a counter to signal when people are
+	using the resource at hand.  Allows multiple readers.
+"
+}
+
 @techreport{Slingwine95
 ,author="John D. Slingwine and Paul E. McKenney"
 ,title="Apparatus and Method for Achieving Reduced Overhead Mutual
@@ -312,6 +373,49 @@ Andrea Arcangeli and Andi Kleen and Orra
 [Viewed June 23, 2004]"
 }
 
+@...ference{Michael02a
+,author="Maged M. Michael"
+,title="Safe Memory Reclamation for Dynamic Lock-Free Objects Using Atomic
+Reads and Writes"
+,Year="2002"
+,Month="August"
+,booktitle="{Proceedings of the 21\textsuperscript{st} Annual ACM
+Symposium on Principles of Distributed Computing}"
+,pages="21-30"
+,annotation="
+	Each thread keeps an array of pointers to items that it is
+	currently referencing.	Sort of an inside-out garbage collection
+	mechanism, but one that requires the accessing code to explicitly
+	state its needs.  Also requires read-side memory barriers on
+	most architectures.
+"
+}
+
+@...ference{Michael02b
+,author="Maged M. Michael"
+,title="High Performance Dynamic Lock-Free Hash Tables and List-Based Sets"
+,Year="2002"
+,Month="August"
+,booktitle="{Proceedings of the 14\textsuperscript{th} Annual ACM
+Symposium on Parallel
+Algorithms and Architecture}"
+,pages="73-82"
+,annotation="
+	Like the title says...
+"
+}
+
+@...roceedings{HerlihyLM02
+,author={Maurice Herlihy and Victor Luchangco and Mark Moir}
+,title="The Repeat Offender Problem: A Mechanism for Supporting Dynamic-Sized,
+Lock-Free Data Structures"
+,booktitle={Proceedings of 16\textsuperscript{th} International
+Symposium on Distributed Computing}
+,year=2002
+,month="October"
+,pages="339-353"
+}
+
 @article{Appavoo03a
 ,author="J. Appavoo and K. Hui and C. A. N. Soules and R. W. Wisniewski and
 D. M. {Da Silva} and O. Krieger and M. A. Auslander and D. J. Edelsohn and
@@ -447,3 +551,111 @@ Oregon Health and Sciences University"
 	Realtime turns into making RCU yet more realtime friendly.
 "
 }
+
+@...ference{ThomasEHart2006a
+,Author="Thomas E. Hart and Paul E. McKenney and Angela Demke Brown"
+,Title="Making Lockless Synchronization Fast: Performance Implications
+of Memory Reclamation"
+,Booktitle="20\textsuperscript{th} {IEEE} International Parallel and
+Distributed Processing Symposium"
+,month="April"
+,year="2006"
+,day="25-29"
+,address="Rhodes, Greece"
+,annotation="
+	Compares QSBR (AKA "classic RCU"), HPBR, EBR, and lock-free
+	reference counting.
+"
+}
+
+@...ference{PaulEMcKenney2006b
+,Author="Paul E. McKenney and Dipankar Sarma and Ingo Molnar and
+Suparna Bhattacharya"
+,Title="Extending RCU for Realtime and Embedded Workloads"
+,Booktitle="{Ottawa Linux Symposium}"
+,Month="July"
+,Year="2006"
+,pages="v2 123-138"
+,note="Available:
+\url{http://www.linuxsymposium.org/2006/view_abstract.php?content_key=184}
+\url{http://www.rdrop.com/users/paulmck/RCU/OLSrtRCU.2006.08.11a.pdf}
+[Viewed January 1, 2007]"
+,annotation="
+	Described how to improve the -rt implementation of realtime RCU.
+"
+}
+
+@...ublished{PaulEMcKenney2006c
+,Author="Paul E. McKenney"
+,Title="Sleepable {RCU}"
+,month="October"
+,day="9"
+,year="2006"
+,note="Available:
+\url{http://lwn.net/Articles/202847/}
+Revised:
+\url{http://www.rdrop.com/users/paulmck/RCU/srcu.2007.01.14a.pdf}
+[Viewed August 21, 2006]"
+,annotation="
+	LWN article introducing SRCU.
+"
+}
+
+@...ublished{RobertOlsson2006a
+,Author="Robert Olsson and Stefan Nilsson"
+,Title="{TRASH}: A dynamic {LC}-trie and hash data structure"
+,month="August"
+,day="18"
+,year="2006"
+,note="Available:
+\url{http://www.nada.kth.se/~snilsson/public/papers/trash/trash.pdf}
+[Viewed February 24, 2007]"
+,annotation="
+	RCU-protected dynamic trie-hash combination.
+"
+}
+
+@...ublished{PaulEMcKenney2007BoostRCU
+,Author="Paul E. McKenney"
+,Title="Priority-Boosting {RCU} Read-Side Critical Sections"
+,month="February"
+,day="5"
+,year="2007"
+,note="Available:
+\url{http://lwn.net/Articles/220677/}
+Revised:
+\url{http://www.rdrop.com/users/paulmck/RCU/RCUbooststate.2007.04.16a.pdf}
+[Viewed September 7, 2007]"
+,annotation="
+	LWN article introducing RCU priority boosting.
+"
+}
+
+@...ublished{ThomasEHart2007a
+,Author="Thomas E. Hart and Paul E. McKenney and Angela Demke Brown and Jonathan Walpole"
+,Title="Performance of memory reclamation for lockless synchronization"
+,journal="J. Parallel Distrib. Comput."
+,year="2007"
+,note="To appear in J. Parallel Distrib. Comput.
+       \url{doi=10.1016/j.jpdc.2007.04.010}"
+,annotation={
+	Compares QSBR (AKA "classic RCU"), HPBR, EBR, and lock-free
+	reference counting.  Journal version of ThomasEHart2006a.
+}
+}
+
+@...ublished{PaulEMcKenney2007QRCUspin
+,Author="Paul E. McKenney"
+,Title="Using Promela and Spin to verify parallel algorithms"
+,month="August"
+,day="1"
+,year="2007"
+,note="Available:
+\url{http://lwn.net/Articles/243851/}
+[Viewed September 8, 2007]"
+,annotation="
+	LWN article describing Promela and spin, and also using Oleg
+	Nesterov's QRCU as an example (with Paul McKenney's fastpath).
+"
+}
+
diff -urpNa -X dontdiff linux-2.6.22-h-boostsleep/Documentation/RCU/torture.txt linux-2.6.22-i-cleanups/Documentation/RCU/torture.txt
--- linux-2.6.22-h-boostsleep/Documentation/RCU/torture.txt	2007-07-08 16:32:17.000000000 -0700
+++ linux-2.6.22-i-cleanups/Documentation/RCU/torture.txt	2007-09-09 17:02:58.000000000 -0700
@@ -37,6 +37,24 @@ nfakewriters	This is the number of RCU f
 		to trigger special cases caused by multiple writers, such as
 		the synchronize_srcu() early return optimization.
 
+preempt_torture	Specifies that torturing of preemptible RCU is to be
+		undertaken, defaults to no such testing.  This test
+		creates a kernel thread that runs at the lowest possible
+		realtime priority, alternating between ten seconds
+		of spinning and a short sleep period.  The goal is
+		to preempt lower-priority RCU readers.	Note that this
+		currently does not fail the full test, but instead simply
+		counts the number of times that a ten-second CPU burst
+		coincides with a stall in grace-period detection.
+
+		Of course, if the grace period advances during a CPU burst,
+		that indicates that no RCU reader was preempted, so the
+		burst ends early in that case.
+
+		Note that such stalls are expected behavior in preemptible
+		RCU implementations when RCU priority boosting is not
+		enabled (PREEMPT_RCU_BOOST=n).
+
 stat_interval	The number of seconds between output of torture
 		statistics (via printk()).  Regardless of the interval,
 		statistics are printed when the module is unloaded.
@@ -46,12 +64,13 @@ stat_interval	The number of seconds betw
 
 shuffle_interval
 		The number of seconds to keep the test threads affinitied
-		to a particular subset of the CPUs.  Used in conjunction
-		with test_no_idle_hz.
+		to a particular subset of the CPUs, defaults to 5 seconds.
+		Used in conjunction with test_no_idle_hz.
 
 test_no_idle_hz	Whether or not to test the ability of RCU to operate in
 		a kernel that disables the scheduling-clock interrupt to
 		idle CPUs.  Boolean parameter, "1" to test, "0" otherwise.
+		Defaults to omitting this test.
 
 torture_type	The type of RCU to test: "rcu" for the rcu_read_lock() API,
 		"rcu_sync" for rcu_read_lock() with synchronous reclamation,
@@ -82,8 +101,6 @@ be evident.  ;-)
 
 The entries are as follows:
 
-o	"ggp": The number of counter flips (or batches) since boot.
-
 o	"rtc": The hexadecimal address of the structure currently visible
 	to readers.
 
@@ -117,8 +134,8 @@ o	"Reader Pipe": Histogram of "ages" of 
 o	"Reader Batch": Another histogram of "ages" of structures seen
 	by readers, but in terms of counter flips (or batches) rather
 	than in terms of grace periods.  The legal number of non-zero
-	entries is again two.  The reason for this separate view is
-	that it is easier to get the third entry to show up in the
+	entries is again two.  The reason for this separate view is that
+	it is sometimes easier to get the third entry to show up in the
 	"Reader Batch" list than in the "Reader Pipe" list.
 
 o	"Free-Block Circulation": Shows the number of torture structures
@@ -145,6 +162,21 @@ of the "old" and "current" counters for 
 "idx" value maps the "old" and "current" values to the underlying array,
 and is useful for debugging.
 
+In addition, preemptible RCU rcutorture runs will report preemption
+stalls:
+
+rcu-torture: rtc: ffffffff88005a40 ver: 17041 tfle: 1 rta: 17041 rtaf: 7904 rtf: 16941 rtmbe: 0
+rcu-torture: Reader Pipe:  975332139 34406 0 0 0 0 0 0 0 0 0
+rcu-torture: Reader Batch:  975349310 17234 0 0 0 0 0 0 0 0 0
+rcu-torture: Free-Block Circulation:  17040 17030 17028 17022 17009 16994 16982 16969 16955 16941 0
+Preemption stalls: 0
+
+The first four lines are as before, and the last line records the number
+of times that grace-period processing stalled during a realtime CPU burst.
+Note that a non-zero value does not -prove- that RCU priority boosting is
+broken, because there are other things that can stall RCU grace-period
+processing.  Here is hoping that someone comes up with a better test!
+
 
 USAGE
 
diff -urpNa -X dontdiff linux-2.6.22-h-boostsleep/kernel/Kconfig.preempt linux-2.6.22-i-cleanups/kernel/Kconfig.preempt
--- linux-2.6.22-h-boostsleep/kernel/Kconfig.preempt	2007-09-08 14:02:29.000000000 -0700
+++ linux-2.6.22-i-cleanups/kernel/Kconfig.preempt	2007-09-08 14:08:31.000000000 -0700
@@ -79,6 +79,7 @@ config CLASSIC_RCU
 config PREEMPT_RCU
 	bool "Preemptible RCU"
 	depends on PREEMPT
+	depends on EXPERIMENTAL
 	help
 	  This option reduces the latency of the kernel by making certain
 	  RCU sections preemptible. Normally RCU code is non-preemptible, if
diff -urpNa -X dontdiff linux-2.6.22-h-boostsleep/kernel/rcupreempt.c linux-2.6.22-i-cleanups/kernel/rcupreempt.c
--- linux-2.6.22-h-boostsleep/kernel/rcupreempt.c	2007-09-08 14:06:27.000000000 -0700
+++ linux-2.6.22-i-cleanups/kernel/rcupreempt.c	2007-09-08 14:08:31.000000000 -0700
@@ -59,7 +59,7 @@
  * only to mediate communication between mainline code and hardware
  * interrupt and NMI handlers.
  */
-#define ORDERED_WRT_IRQ(x) (*(volatile typeof(x) *)&(x))
+#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
 
 /*
  * PREEMPT_RCU data structures.
@@ -358,7 +358,7 @@ static void init_rcu_boost_early(void)
 static inline struct rcu_boost_dat *rcu_rbd_new(void)
 {
 	int cpu = smp_processor_id();  /* locks used, so preemption OK. */
-	int idx = ORDERED_WRT_IRQ(rcu_boost_idx);
+	int idx = ACCESS_ONCE(rcu_boost_idx);
 
 	if (unlikely(idx < 0))
 		return NULL;
@@ -810,7 +810,7 @@ void __rcu_read_lock(void)
 	struct task_struct *me = current;
 	int nesting;
 
-	nesting = ORDERED_WRT_IRQ(me->rcu_read_lock_nesting);
+	nesting = ACCESS_ONCE(me->rcu_read_lock_nesting);
 	if (nesting != 0) {
 
 		/* An earlier rcu_read_lock() covers us, just count it. */
@@ -835,9 +835,9 @@ void __rcu_read_lock(void)
 		 * casts to prevent the compiler from reordering.
 		 */
 
-		idx = ORDERED_WRT_IRQ(rcu_ctrlblk.completed) & 0x1;
+		idx = ACCESS_ONCE(rcu_ctrlblk.completed) & 0x1;
 		smp_read_barrier_depends();  /* @@@@ might be unneeded */
-		ORDERED_WRT_IRQ(__get_cpu_var(rcu_flipctr)[idx])++;
+		ACCESS_ONCE(__get_cpu_var(rcu_flipctr)[idx])++;
 
 		/*
 		 * Now that the per-CPU counter has been incremented, we
@@ -847,7 +847,7 @@ void __rcu_read_lock(void)
 		 * of the need to increment the per-CPU counter.
 		 */
 
-		ORDERED_WRT_IRQ(me->rcu_read_lock_nesting) = nesting + 1;
+		ACCESS_ONCE(me->rcu_read_lock_nesting) = nesting + 1;
 
 		/*
 		 * Now that we have preventing any NMIs from storing
@@ -856,7 +856,7 @@ void __rcu_read_lock(void)
 		 * rcu_read_unlock().
 		 */
 
-		ORDERED_WRT_IRQ(me->rcu_flipctr_idx) = idx;
+		ACCESS_ONCE(me->rcu_flipctr_idx) = idx;
 		local_irq_restore(flags);
 	}
 }
@@ -868,7 +868,7 @@ void __rcu_read_unlock(void)
 	struct task_struct *me = current;
 	int nesting;
 
-	nesting = ORDERED_WRT_IRQ(me->rcu_read_lock_nesting);
+	nesting = ACCESS_ONCE(me->rcu_read_lock_nesting);
 	if (nesting > 1) {
 
 		/*
@@ -908,7 +908,7 @@ void __rcu_read_unlock(void)
 		 * DEC Alpha.
 		 */
 
-		idx = ORDERED_WRT_IRQ(me->rcu_flipctr_idx);
+		idx = ACCESS_ONCE(me->rcu_flipctr_idx);
 		smp_read_barrier_depends();  /* @@@ Needed??? */
 
 		/*
@@ -917,7 +917,7 @@ void __rcu_read_unlock(void)
 		 * After this, any interrupts or NMIs will increment and
 		 * decrement the per-CPU counters.
 		 */
-		ORDERED_WRT_IRQ(me->rcu_read_lock_nesting) = nesting - 1;
+		ACCESS_ONCE(me->rcu_read_lock_nesting) = nesting - 1;
 
 		/*
 		 * It is now safe to decrement this task's nesting count.
@@ -928,7 +928,7 @@ void __rcu_read_unlock(void)
 		 * but that is OK, since we have already fetched it.
 		 */
 
-		ORDERED_WRT_IRQ(__get_cpu_var(rcu_flipctr)[idx])--;
+		ACCESS_ONCE(__get_cpu_var(rcu_flipctr)[idx])--;
 
 		rcu_read_unlock_unboost();
 
@@ -1123,7 +1123,7 @@ rcu_try_flip_waitmb(void)
 /*
  * Attempt a single flip of the counters.  Remember, a single flip does
  * -not- constitute a grace period.  Instead, the interval between
- * at least three consecutive flips is a grace period.
+ * at least GP_STAGES+2 consecutive flips is a grace period.
  *
  * If anyone is nuts enough to run this CONFIG_PREEMPT_RCU implementation
  * on a large SMP, they might want to use a hierarchical organization of
-
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