[PATCH] RCU torture-testing kernel module

This patch is a rewrite of the one submitted on October 1st, using modules
(http://marc.theaimsgroup.com/?l=linux-kernel&m=112819093522998&w=2).

This rewrite adds a tristate CONFIG_RCU_TORTURE_TEST, which enables an
intense torture test of the RCU infratructure.  This is needed due to the
continued changes to the RCU infrastructure to accommodate dynamic ticks,
CPU hotplug, realtime, and so on.  Most of the code is in a separate file
that is compiled only if the CONFIG variable is set.  Documentation on how
to run the test and interpret the output is also included.

This code has been tested on i386 and ppc64, and an earlier version of the
code has received extensive testing on a number of architectures as part of
the PREEMPT_RT patchset.
Signed-off-by: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

Documentation/RCU/torture.txt

+RCU Torture Test Operation
+
+
+CONFIG_RCU_TORTURE_TEST
+
+The CONFIG_RCU_TORTURE_TEST config option is available for all RCU
+implementations.  It creates an rcutorture kernel module that can
+be loaded to run a torture test.  The test periodically outputs
+status messages via printk(), which can be examined via the dmesg
+command (perhaps grepping for "rcutorture").  The test is started
+when the module is loaded, and stops when the module is unloaded.
+
+However, actually setting this config option to "y" results in the system
+running the test immediately upon boot, and ending only when the system
+is taken down.  Normally, one will instead want to build the system
+with CONFIG_RCU_TORTURE_TEST=m and to use modprobe and rmmod to control
+the test, perhaps using a script similar to the one shown at the end of
+this document.  Note that you will need CONFIG_MODULE_UNLOAD in order
+to be able to end the test.
+
+
+MODULE PARAMETERS
+
+This module has the following parameters:
+
+nreaders	This is the number of RCU reading threads supported.
+		The default is twice the number of CPUs.  Why twice?
+		To properly exercise RCU implementations with preemptible
+		read-side critical sections.
+
+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.
+		Setting the interval to zero causes the statistics to
+		be printed -only- when the module is unloaded, and this
+		is the default.
+
+verbose		Enable debug printk()s.  Default is disabled.
+
+
+OUTPUT
+
+The statistics output is as follows:
+
+	rcutorture: --- Start of test: nreaders=16 stat_interval=0 verbose=0
+	rcutorture: rtc: 0000000000000000 ver: 1916 tfle: 0 rta: 1916 rtaf: 0 rtf: 1915
+	rcutorture: Reader Pipe:  1466408 9747 0 0 0 0 0 0 0 0 0
+	rcutorture: Reader Batch:  1464477 11678 0 0 0 0 0 0 0 0
+	rcutorture: Free-Block Circulation:  1915 1915 1915 1915 1915 1915 1915 1915 1915 1915 0
+	rcutorture: --- End of test
+
+The command "dmesg | grep rcutorture:" will extract this information on
+most systems.  On more esoteric configurations, it may be necessary to
+use other commands to access the output of the printk()s used by
+the RCU torture test.  The printk()s use KERN_ALERT, so they should
+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.
+
+o	"ver": The number of times since boot that the rcutw writer task
+	has changed the structure visible to readers.
+
+o	"tfle": If non-zero, indicates that the "torture freelist"
+	containing structure to be placed into the "rtc" area is empty.
+	This condition is important, since it can fool you into thinking
+	that RCU is working when it is not.  :-/
+
+o	"rta": Number of structures allocated from the torture freelist.
+
+o	"rtaf": Number of allocations from the torture freelist that have
+	failed due to the list being empty.
+
+o	"rtf": Number of frees into the torture freelist.
+
+o	"Reader Pipe": Histogram of "ages" of structures seen by readers.
+	If any entries past the first two are non-zero, RCU is broken.
+	And rcutorture prints the error flag string "!!!" to make sure
+	you notice.  The age of a newly allocated structure is zero,
+	it becomes one when removed from reader visibility, and is
+	incremented once per grace period subsequently -- and is freed
+	after passing through (RCU_TORTURE_PIPE_LEN-2) grace periods.
+
+	The output displayed above was taken from a correctly working
+	RCU.  If you want to see what it looks like when broken, break
+	it yourself.  ;-)
+
+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
+	"Reader Batch" list than in the "Reader Pipe" list.
+
+o	"Free-Block Circulation": Shows the number of torture structures
+	that have reached a given point in the pipeline.  The first element
+	should closely correspond to the number of structures allocated,
+	the second to the number that have been removed from reader view,
+	and all but the last remaining to the corresponding number of
+	passes through a grace period.  The last entry should be zero,
+	as it is only incremented if a torture structure's counter
+	somehow gets incremented farther than it should.
+
+
+USAGE
+
+The following script may be used to torture RCU:
+
+	#!/bin/sh
+
+	modprobe rcutorture
+	sleep 100
+	rmmod rcutorture
+	dmesg | grep rcutorture:
+
+The output can be manually inspected for the error flag of "!!!".
+One could of course create a more elaborate script that automatically
+checked for such errors.

include/linux/rcupdate.h

@@ -275,6 +275,7 @@ static inline int rcu_pending(int cpu)
 extern void rcu_init(void);
 extern void rcu_check_callbacks(int cpu, int user);
 extern void rcu_restart_cpu(int cpu);
+extern long rcu_batches_completed(void);
 
 /* Exported interfaces */
 extern void FASTCALL(call_rcu(struct rcu_head *head, 

kernel/Makefile

@@ -31,6 +31,7 @@ obj-$(CONFIG_DETECT_SOFTLOCKUP) += softlockup.o
 obj-$(CONFIG_GENERIC_HARDIRQS) += irq/
 obj-$(CONFIG_CRASH_DUMP) += crash_dump.o
 obj-$(CONFIG_SECCOMP) += seccomp.o
+obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
 
 ifneq ($(CONFIG_SCHED_NO_NO_OMIT_FRAME_POINTER),y)
 # According to Alan Modra <alan@linuxcare.com.au>, the -fno-omit-frame-pointer is

kernel/rcupdate.c

@@ -153,6 +153,15 @@ void fastcall call_rcu_bh(struct rcu_head *head,
 	local_irq_restore(flags);
 }
 
+/*
+ * Return the number of RCU batches processed thus far.  Useful
+ * for debug and statistics.
+ */
+long rcu_batches_completed(void)
+{
+	return rcu_ctrlblk.completed;
+}
+
 /*
  * Invoke the completed RCU callbacks. They are expected to be in
  * a per-cpu list.
@@ -501,6 +510,7 @@ void synchronize_kernel(void)
 }
 
 module_param(maxbatch, int, 0);
+EXPORT_SYMBOL_GPL(rcu_batches_completed);
 EXPORT_SYMBOL(call_rcu);  /* WARNING: GPL-only in April 2006. */
 EXPORT_SYMBOL(call_rcu_bh);  /* WARNING: GPL-only in April 2006. */
 EXPORT_SYMBOL_GPL(synchronize_rcu);

kernel/rcutorture.c

+/*
+ * Read-Copy Update /proc-based torture test facility
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *
+ * Copyright (C) IBM Corporation, 2005
+ *
+ * Authors: Paul E. McKenney <paulmck@us.ibm.com>
+ *
+ * See also:  Documentation/RCU/torture.txt
+ */
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/kthread.h>
+#include <linux/err.h>
+#include <linux/spinlock.h>
+#include <linux/smp.h>
+#include <linux/rcupdate.h>
+#include <linux/interrupt.h>
+#include <linux/sched.h>
+#include <asm/atomic.h>
+#include <linux/bitops.h>
+#include <linux/module.h>
+#include <linux/completion.h>
+#include <linux/moduleparam.h>
+#include <linux/percpu.h>
+#include <linux/notifier.h>
+#include <linux/rcuref.h>
+#include <linux/cpu.h>
+#include <linux/random.h>
+#include <linux/delay.h>
+#include <linux/byteorder/swabb.h>
+#include <linux/stat.h>
+
+MODULE_LICENSE("GPL");
+
+static int nreaders = -1;	/* # reader threads, defaults to 4*ncpus */
+static int stat_interval = 0;	/* Interval between stats, in seconds. */
+				/*  Defaults to "only at end of test". */
+static int verbose = 0;		/* Print more debug info. */
+
+MODULE_PARM(nreaders, "i");
+MODULE_PARM_DESC(nreaders, "Number of RCU reader threads");
+MODULE_PARM(stat_interval, "i");
+MODULE_PARM_DESC(stat_interval, "Number of seconds between stats printk()s");
+MODULE_PARM(verbose, "i");
+MODULE_PARM_DESC(verbose, "Enable verbose debugging printk()s");
+#define TORTURE_FLAG "rcutorture: "
+#define PRINTK_STRING(s) \
+	do { printk(KERN_ALERT TORTURE_FLAG s "\n"); } while (0)
+#define VERBOSE_PRINTK_STRING(s) \
+	do { if (verbose) printk(KERN_ALERT TORTURE_FLAG s "\n"); } while (0)
+#define VERBOSE_PRINTK_ERRSTRING(s) \
+	do { if (verbose) printk(KERN_ALERT TORTURE_FLAG "!!! " s "\n"); } while (0)
+
+static char printk_buf[4096];
+
+static int nrealreaders;
+static struct task_struct *writer_task;
+static struct task_struct **reader_tasks;
+static struct task_struct *stats_task;
+
+#define RCU_TORTURE_PIPE_LEN 10
+
+struct rcu_torture {
+	struct rcu_head rtort_rcu;
+	int rtort_pipe_count;
+	struct list_head rtort_free;
+};
+
+static int fullstop = 0;	/* stop generating callbacks at test end. */
+static LIST_HEAD(rcu_torture_freelist);
+static struct rcu_torture *rcu_torture_current = NULL;
+static long rcu_torture_current_version = 0;
+static struct rcu_torture rcu_tortures[10 * RCU_TORTURE_PIPE_LEN];
+static DEFINE_SPINLOCK(rcu_torture_lock);
+static DEFINE_PER_CPU(long [RCU_TORTURE_PIPE_LEN + 1], rcu_torture_count) =
+	{ 0 };
+static DEFINE_PER_CPU(long [RCU_TORTURE_PIPE_LEN + 1], rcu_torture_batch) =
+	{ 0 };
+static atomic_t rcu_torture_wcount[RCU_TORTURE_PIPE_LEN + 1];
+atomic_t n_rcu_torture_alloc;
+atomic_t n_rcu_torture_alloc_fail;
+atomic_t n_rcu_torture_free;
+
+/*
+ * Allocate an element from the rcu_tortures pool.
+ */
+struct rcu_torture *
+rcu_torture_alloc(void)
+{
+	struct list_head *p;
+
+	spin_lock(&rcu_torture_lock);
+	if (list_empty(&rcu_torture_freelist)) {
+		atomic_inc(&n_rcu_torture_alloc_fail);
+		spin_unlock(&rcu_torture_lock);
+		return NULL;
+	}
+	atomic_inc(&n_rcu_torture_alloc);
+	p = rcu_torture_freelist.next;
+	list_del_init(p);
+	spin_unlock(&rcu_torture_lock);
+	return container_of(p, struct rcu_torture, rtort_free);
+}
+
+/*
+ * Free an element to the rcu_tortures pool.
+ */
+static void
+rcu_torture_free(struct rcu_torture *p)
+{
+	atomic_inc(&n_rcu_torture_free);
+	spin_lock(&rcu_torture_lock);
+	list_add_tail(&p->rtort_free, &rcu_torture_freelist);
+	spin_unlock(&rcu_torture_lock);
+}
+
+static void
+rcu_torture_cb(struct rcu_head *p)
+{
+	int i;
+	struct rcu_torture *rp = container_of(p, struct rcu_torture, rtort_rcu);
+
+	if (fullstop) {
+		/* Test is ending, just drop callbacks on the floor. */
+		/* The next initialization will pick up the pieces. */
+		return;
+	}
+	i = rp->rtort_pipe_count;
+	if (i > RCU_TORTURE_PIPE_LEN)
+		i = RCU_TORTURE_PIPE_LEN;
+	atomic_inc(&rcu_torture_wcount[i]);
+	if (++rp->rtort_pipe_count >= RCU_TORTURE_PIPE_LEN)
+		rcu_torture_free(rp);
+	else
+		call_rcu(p, rcu_torture_cb);
+}
+
+struct rcu_random_state {
+	unsigned long rrs_state;
+	unsigned long rrs_count;
+};
+
+#define RCU_RANDOM_MULT 39916801  /* prime */
+#define RCU_RANDOM_ADD	479001701 /* prime */
+#define RCU_RANDOM_REFRESH 10000
+
+#define DEFINE_RCU_RANDOM(name) struct rcu_random_state name = { 0, 0 }
+
+/*
+ * Crude but fast random-number generator.  Uses a linear congruential
+ * generator, with occasional help from get_random_bytes().
+ */
+static long
+rcu_random(struct rcu_random_state *rrsp)
+{
+	long refresh;
+
+	if (--rrsp->rrs_count < 0) {
+		get_random_bytes(&refresh, sizeof(refresh));
+		rrsp->rrs_state += refresh;
+		rrsp->rrs_count = RCU_RANDOM_REFRESH;
+	}
+	rrsp->rrs_state = rrsp->rrs_state * RCU_RANDOM_MULT + RCU_RANDOM_ADD;
+	return swahw32(rrsp->rrs_state);
+}
+
+/*
+ * RCU torture writer kthread.  Repeatedly substitutes a new structure
+ * for that pointed to by rcu_torture_current, freeing the old structure
+ * after a series of grace periods (the "pipeline").
+ */
+static int
+rcu_torture_writer(void *arg)
+{
+	int i;
+	long oldbatch = rcu_batches_completed();
+	struct rcu_torture *rp;
+	struct rcu_torture *old_rp;
+	static DEFINE_RCU_RANDOM(rand);
+
+	VERBOSE_PRINTK_STRING("rcu_torture_writer task started");
+	do {
+		schedule_timeout_uninterruptible(1);
+		if (rcu_batches_completed() == oldbatch)
+			continue;
+		if ((rp = rcu_torture_alloc()) == NULL)
+			continue;
+		rp->rtort_pipe_count = 0;
+		udelay(rcu_random(&rand) & 0x3ff);
+		old_rp = rcu_torture_current;
+		rcu_assign_pointer(rcu_torture_current, rp);
+		smp_wmb();
+		if (old_rp != NULL) {
+			i = old_rp->rtort_pipe_count;
+			if (i > RCU_TORTURE_PIPE_LEN)
+				i = RCU_TORTURE_PIPE_LEN;
+			atomic_inc(&rcu_torture_wcount[i]);
+			old_rp->rtort_pipe_count++;
+			call_rcu(&old_rp->rtort_rcu, rcu_torture_cb);
+		}
+		rcu_torture_current_version++;
+		oldbatch = rcu_batches_completed();
+	} while (!kthread_should_stop() && !fullstop);
+	VERBOSE_PRINTK_STRING("rcu_torture_writer task stopping");
+	while (!kthread_should_stop())
+		schedule_timeout_uninterruptible(1);
+	return 0;
+}
+
+/*
+ * RCU torture reader kthread.  Repeatedly dereferences rcu_torture_current,
+ * incrementing the corresponding element of the pipeline array.  The
+ * counter in the element should never be greater than 1, otherwise, the
+ * RCU implementation is broken.
+ */
+static int
+rcu_torture_reader(void *arg)
+{
+	int completed;
+	DEFINE_RCU_RANDOM(rand);
+	struct rcu_torture *p;
+	int pipe_count;
+
+	VERBOSE_PRINTK_STRING("rcu_torture_reader task started");
+	do {
+		rcu_read_lock();
+		completed = rcu_batches_completed();
+		p = rcu_dereference(rcu_torture_current);
+		if (p == NULL) {
+			/* Wait for rcu_torture_writer to get underway */
+			rcu_read_unlock();
+			schedule_timeout_interruptible(HZ);
+			continue;
+		}
+		udelay(rcu_random(&rand) & 0x7f);
+		preempt_disable();
+		pipe_count = p->rtort_pipe_count;
+		if (pipe_count > RCU_TORTURE_PIPE_LEN) {
+			/* Should not happen, but... */
+			pipe_count = RCU_TORTURE_PIPE_LEN;
+		}
+		++__get_cpu_var(rcu_torture_count)[pipe_count];
+		completed = rcu_batches_completed() - completed;
+		if (completed > RCU_TORTURE_PIPE_LEN) {
+			/* Should not happen, but... */
+			completed = RCU_TORTURE_PIPE_LEN;
+		}
+		++__get_cpu_var(rcu_torture_batch)[completed];
+		preempt_enable();
+		rcu_read_unlock();
+		schedule();
+	} while (!kthread_should_stop() && !fullstop);
+	VERBOSE_PRINTK_STRING("rcu_torture_reader task stopping");
+	while (!kthread_should_stop())
+		schedule_timeout_uninterruptible(1);
+	return 0;
+}
+
+/*
+ * Create an RCU-torture statistics message in the specified buffer.
+ */
+static int
+rcu_torture_printk(char *page)
+{
+	int cnt = 0;
+	int cpu;
+	int i;
+	long pipesummary[RCU_TORTURE_PIPE_LEN + 1] = { 0 };
+	long batchsummary[RCU_TORTURE_PIPE_LEN + 1] = { 0 };
+
+	for_each_cpu(cpu) {
+		for (i = 0; i < RCU_TORTURE_PIPE_LEN + 1; i++) {
+			pipesummary[i] += per_cpu(rcu_torture_count, cpu)[i];
+			batchsummary[i] += per_cpu(rcu_torture_batch, cpu)[i];
+		}
+	}
+	for (i = RCU_TORTURE_PIPE_LEN - 1; i >= 0; i--) {
+		if (pipesummary[i] != 0)
+			break;
+	}
+	cnt += sprintf(&page[cnt], "rcutorture: ");
+	cnt += sprintf(&page[cnt],
+		       "rtc: %p ver: %ld tfle: %d rta: %d rtaf: %d rtf: %d",
+		       rcu_torture_current,
+		       rcu_torture_current_version,
+		       list_empty(&rcu_torture_freelist),
+		       atomic_read(&n_rcu_torture_alloc),
+		       atomic_read(&n_rcu_torture_alloc_fail),
+		       atomic_read(&n_rcu_torture_free));
+	cnt += sprintf(&page[cnt], "\nrcutorture: ");
+	if (i > 1)
+		cnt += sprintf(&page[cnt], "!!! ");
+	cnt += sprintf(&page[cnt], "Reader Pipe: ");
+	for (i = 0; i < RCU_TORTURE_PIPE_LEN + 1; i++)
+		cnt += sprintf(&page[cnt], " %ld", pipesummary[i]);
+	cnt += sprintf(&page[cnt], "\nrcutorture: ");
+	cnt += sprintf(&page[cnt], "Reader Batch: ");
+	for (i = 0; i < RCU_TORTURE_PIPE_LEN; i++)
+		cnt += sprintf(&page[cnt], " %ld", batchsummary[i]);
+	cnt += sprintf(&page[cnt], "\nrcutorture: ");
+	cnt += sprintf(&page[cnt], "Free-Block Circulation: ");
+	for (i = 0; i < RCU_TORTURE_PIPE_LEN + 1; i++) {
+		cnt += sprintf(&page[cnt], " %d",
+			       atomic_read(&rcu_torture_wcount[i]));
+	}
+	cnt += sprintf(&page[cnt], "\n");
+	return cnt;
+}
+
+/*
+ * Print torture statistics.  Caller must ensure that there is only
+ * one call to this function at a given time!!!  This is normally
+ * accomplished by relying on the module system to only have one copy
+ * of the module loaded, and then by giving the rcu_torture_stats
+ * kthread full control (or the init/cleanup functions when rcu_torture_stats
+ * thread is not running).
+ */
+static void
+rcu_torture_stats_print(void)
+{
+	int cnt;
+
+	cnt = rcu_torture_printk(printk_buf);
+	printk(KERN_ALERT "%s", printk_buf);
+}
+
+/*
+ * Periodically prints torture statistics, if periodic statistics printing
+ * was specified via the stat_interval module parameter.
+ *
+ * No need to worry about fullstop here, since this one doesn't reference
+ * volatile state or register callbacks.
+ */
+static int
+rcu_torture_stats(void *arg)
+{
+	VERBOSE_PRINTK_STRING("rcu_torture_stats task started");
+	do {
+		schedule_timeout_interruptible(stat_interval * HZ);
+		rcu_torture_stats_print();
+	} while (!kthread_should_stop());
+	VERBOSE_PRINTK_STRING("rcu_torture_stats task stopping");
+	return 0;
+}
+
+static void
+rcu_torture_cleanup(void)
+{
+	int i;
+
+	fullstop = 1;
+	if (writer_task != NULL) {
+		VERBOSE_PRINTK_STRING("Stopping rcu_torture_writer task");
+		kthread_stop(writer_task);
+	}
+	writer_task = NULL;
+
+	if (reader_tasks != NULL) {
+		for (i = 0; i < nrealreaders; i++) {
+			if (reader_tasks[i] != NULL) {
+				VERBOSE_PRINTK_STRING(
+					"Stopping rcu_torture_reader task");
+				kthread_stop(reader_tasks[i]);
+			}
+			reader_tasks[i] = NULL;
+		}
+		kfree(reader_tasks);
+		reader_tasks = NULL;
+	}
+	rcu_torture_current = NULL;
+
+	if (stats_task != NULL) {
+		VERBOSE_PRINTK_STRING("Stopping rcu_torture_stats task");
+		kthread_stop(stats_task);
+	}
+	stats_task = NULL;
+
+	/* Wait for all RCU callbacks to fire.  */
+
+	for (i = 0; i < RCU_TORTURE_PIPE_LEN; i++)
+		synchronize_rcu();
+	rcu_torture_stats_print();  /* -After- the stats thread is stopped! */
+	PRINTK_STRING("--- End of test");
+}
+
+static int
+rcu_torture_init(void)
+{
+	int i;
+	int cpu;
+	int firsterr = 0;
+
+	/* Process args and tell the world that the torturer is on the job. */
+
+	if (nreaders >= 0)
+		nrealreaders = nreaders;
+	else
+		nrealreaders = 2 * num_online_cpus();
+	printk(KERN_ALERT TORTURE_FLAG
+	       "--- Start of test: nreaders=%d stat_interval=%d verbose=%d\n",
+	       nrealreaders, stat_interval, verbose);
+	fullstop = 0;
+
+	/* Set up the freelist. */
+
+	INIT_LIST_HEAD(&rcu_torture_freelist);
+	for (i = 0; i < sizeof(rcu_tortures) / sizeof(rcu_tortures[0]); i++) {
+		list_add_tail(&rcu_tortures[i].rtort_free,
+			      &rcu_torture_freelist);
+	}
+
+	/* Initialize the statistics so that each run gets its own numbers. */
+
+	rcu_torture_current = NULL;
+	rcu_torture_current_version = 0;
+	atomic_set(&n_rcu_torture_alloc, 0);
+	atomic_set(&n_rcu_torture_alloc_fail, 0);
+	atomic_set(&n_rcu_torture_free, 0);
+	for (i = 0; i < RCU_TORTURE_PIPE_LEN + 1; i++)
+		atomic_set(&rcu_torture_wcount[i], 0);
+	for_each_cpu(cpu) {
+		for (i = 0; i < RCU_TORTURE_PIPE_LEN + 1; i++) {
+			per_cpu(rcu_torture_count, cpu)[i] = 0;
+			per_cpu(rcu_torture_batch, cpu)[i] = 0;
+		}
+	}
+
+	/* Start up the kthreads. */
+
+	VERBOSE_PRINTK_STRING("Creating rcu_torture_writer task");
+	writer_task = kthread_run(rcu_torture_writer, NULL,
+				  "rcu_torture_writer");
+	if (IS_ERR(writer_task)) {
+		firsterr = PTR_ERR(writer_task);
+		VERBOSE_PRINTK_ERRSTRING("Failed to create writer");
+		writer_task = NULL;
+		goto unwind;
+	}
+	reader_tasks = kmalloc(nrealreaders * sizeof(reader_tasks[0]),
+			       GFP_KERNEL);
+	if (reader_tasks == NULL) {
+		VERBOSE_PRINTK_ERRSTRING("out of memory");
+		firsterr = -ENOMEM;
+		goto unwind;
+	}
+	for (i = 0; i < nrealreaders; i++) {
+		VERBOSE_PRINTK_STRING("Creating rcu_torture_reader task");
+		reader_tasks[i] = kthread_run(rcu_torture_reader, NULL,
+					      "rcu_torture_reader");
+		if (IS_ERR(reader_tasks[i])) {
+			firsterr = PTR_ERR(reader_tasks[i]);
+			VERBOSE_PRINTK_ERRSTRING("Failed to create reader");
+			reader_tasks[i] = NULL;
+			goto unwind;
+		}
+	}
+	if (stat_interval > 0) {
+		VERBOSE_PRINTK_STRING("Creating rcu_torture_stats task");
+		stats_task = kthread_run(rcu_torture_stats, NULL,
+					"rcu_torture_stats");
+		if (IS_ERR(stats_task)) {
+			firsterr = PTR_ERR(stats_task);
+			VERBOSE_PRINTK_ERRSTRING("Failed to create stats");
+			stats_task = NULL;
+			goto unwind;
+		}
+	}
+	return 0;
+
+unwind:
+	rcu_torture_cleanup();
+	return firsterr;
+}
+
+module_init(rcu_torture_init);
+module_exit(rcu_torture_cleanup);

lib/Kconfig.debug

@@ -168,6 +168,14 @@ config DEBUG_FS
 
 	  If unsure, say N.
 
+config DEBUG_VM
+	bool "Debug VM"
+	depends on DEBUG_KERNEL
+	help
+	  Enable this to debug the virtual-memory system.
+
+	  If unsure, say N.
+
 config FRAME_POINTER
 	bool "Compile the kernel with frame pointers"
 	depends on DEBUG_KERNEL && (X86 || CRIS || M68K || M68KNOMMU || FRV || UML)
@@ -178,3 +186,16 @@ config FRAME_POINTER
 	  some architectures or if you use external debuggers.
 	  If you don't debug the kernel, you can say N.
 
+config RCU_TORTURE_TEST
+	tristate "torture tests for RCU"
+	depends on DEBUG_KERNEL
+	default n
+	help
+	  This option provides a kernel module that runs torture tests
+	  on the RCU infrastructure.  The kernel module may be built
+	  after the fact on the running kernel to be tested, if desired.
+
+	  Say Y here if you want RCU torture tests to start automatically
+	  at boot time (you probably don't).
+	  Say M if you want the RCU torture tests to build as a module.
+	  Say N if you are unsure.

mm/mmap.c

@@ -1840,7 +1840,7 @@ asmlinkage long sys_munmap(unsigned long addr, size_t len)
 
 static inline void verify_mm_writelocked(struct mm_struct *mm)
 {
-#ifdef CONFIG_DEBUG_KERNEL
+#ifdef CONFIG_DEBUG_VM
 	if (unlikely(down_read_trylock(&mm->mmap_sem))) {
 		WARN_ON(1);
 		up_read(&mm->mmap_sem);