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authorPaul E. McKenney <paulmck@us.ibm.com>2006-10-04 02:17:02 -0700
committerLinus Torvalds <torvalds@g5.osdl.org>2006-10-04 07:55:30 -0700
commit621934ee7ed5b073c7fd638b347e632c53572761 (patch)
tree5722f9cda22c099ad60545f963410dcbc762ee65
parent95d77884c77beed676036d2f74d10b470a483c63 (diff)
downloadlinux-linaro-stable-621934ee7ed5b073c7fd638b347e632c53572761.tar.gz
[PATCH] srcu-3: RCU variant permitting read-side blocking
Updated patch adding a variant of RCU that permits sleeping in read-side critical sections. SRCU is as follows: o Each use of SRCU creates its own srcu_struct, and each srcu_struct has its own set of grace periods. This is critical, as it prevents one subsystem with a blocking reader from holding up SRCU grace periods for other subsystems. o The SRCU primitives (srcu_read_lock(), srcu_read_unlock(), and synchronize_srcu()) all take a pointer to a srcu_struct. o The SRCU primitives must be called from process context. o srcu_read_lock() returns an int that must be passed to the matching srcu_read_unlock(). Realtime RCU avoids the need for this by storing the state in the task struct, but SRCU needs to allow a given code path to pass through multiple SRCU domains -- storing state in the task struct would therefore require either arbitrary space in the task struct or arbitrary limits on SRCU nesting. So I kicked the state-storage problem up to the caller. Of course, it is not permitted to call synchronize_srcu() while in an SRCU read-side critical section. o There is no call_srcu(). It would not be hard to implement one, but it seems like too easy a way to OOM the system. (Hey, we have enough trouble with call_rcu(), which does -not- permit readers to sleep!!!) So, if you want it, please tell me why... [josht@us.ibm.com: sparse notation] Signed-off-by: Paul E. McKenney <paulmck@us.ibm.com> Signed-off-by: Josh Triplett <josh@freedesktop.org> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
-rw-r--r--Documentation/RCU/checklist.txt38
-rw-r--r--Documentation/RCU/rcu.txt3
-rw-r--r--Documentation/RCU/whatisRCU.txt3
-rw-r--r--include/linux/srcu.h49
-rw-r--r--kernel/Makefile2
-rw-r--r--kernel/srcu.c257
6 files changed, 350 insertions, 2 deletions
diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index 1d50cf0c905e..f4dffadbcb00 100644
--- a/Documentation/RCU/checklist.txt
+++ b/Documentation/RCU/checklist.txt
@@ -221,3 +221,41 @@ over a rather long period of time, but improvements are always welcome!
disable irq on a given acquisition of that lock will result in
deadlock as soon as the RCU callback happens to interrupt that
acquisition's critical section.
+
+13. SRCU (srcu_read_lock(), srcu_read_unlock(), and synchronize_srcu())
+ may only be invoked from process context. Unlike other forms of
+ RCU, it -is- permissible to block in an SRCU read-side critical
+ section (demarked by srcu_read_lock() and srcu_read_unlock()),
+ hence the "SRCU": "sleepable RCU". Please note that if you
+ don't need to sleep in read-side critical sections, you should
+ be using RCU rather than SRCU, because RCU is almost always
+ faster and easier to use than is SRCU.
+
+ Also unlike other forms of RCU, explicit initialization
+ and cleanup is required via init_srcu_struct() and
+ cleanup_srcu_struct(). These are passed a "struct srcu_struct"
+ that defines the scope of a given SRCU domain. Once initialized,
+ the srcu_struct is passed to srcu_read_lock(), srcu_read_unlock()
+ and synchronize_srcu(). A given synchronize_srcu() waits only
+ for SRCU read-side critical sections governed by srcu_read_lock()
+ and srcu_read_unlock() calls that have been passd the same
+ srcu_struct. This property is what makes sleeping read-side
+ critical sections tolerable -- a given subsystem delays only
+ its own updates, not those of other subsystems using SRCU.
+ Therefore, SRCU is less prone to OOM the system than RCU would
+ be if RCU's read-side critical sections were permitted to
+ sleep.
+
+ The ability to sleep in read-side critical sections does not
+ come for free. First, corresponding srcu_read_lock() and
+ srcu_read_unlock() calls must be passed the same srcu_struct.
+ Second, grace-period-detection overhead is amortized only
+ over those updates sharing a given srcu_struct, rather than
+ being globally amortized as they are for other forms of RCU.
+ Therefore, SRCU should be used in preference to rw_semaphore
+ only in extremely read-intensive situations, or in situations
+ requiring SRCU's read-side deadlock immunity or low read-side
+ realtime latency.
+
+ Note that, rcu_assign_pointer() and rcu_dereference() relate to
+ SRCU just as they do to other forms of RCU.
diff --git a/Documentation/RCU/rcu.txt b/Documentation/RCU/rcu.txt
index 02e27bf1d365..f84407cba816 100644
--- a/Documentation/RCU/rcu.txt
+++ b/Documentation/RCU/rcu.txt
@@ -45,7 +45,8 @@ o How can I see where RCU is currently used in the Linux kernel?
Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
"rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh",
- "synchronize_rcu", and "synchronize_net".
+ "srcu_read_lock", "srcu_read_unlock", "synchronize_rcu",
+ "synchronize_net", and "synchronize_srcu".
o What guidelines should I follow when writing code that uses RCU?
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 820fee236967..e0d6d99b8f9b 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -778,6 +778,8 @@ Markers for RCU read-side critical sections:
rcu_read_unlock
rcu_read_lock_bh
rcu_read_unlock_bh
+ srcu_read_lock
+ srcu_read_unlock
RCU pointer/list traversal:
@@ -804,6 +806,7 @@ RCU grace period:
synchronize_net
synchronize_sched
synchronize_rcu
+ synchronize_srcu
call_rcu
call_rcu_bh
diff --git a/include/linux/srcu.h b/include/linux/srcu.h
new file mode 100644
index 000000000000..947fdab2ddb0
--- /dev/null
+++ b/include/linux/srcu.h
@@ -0,0 +1,49 @@
+/*
+ * Sleepable Read-Copy Update mechanism for mutual exclusion
+ *
+ * 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, 2006
+ *
+ * Author: Paul McKenney <paulmck@us.ibm.com>
+ *
+ * For detailed explanation of Read-Copy Update mechanism see -
+ * Documentation/RCU/ *.txt
+ *
+ */
+
+struct srcu_struct_array {
+ int c[2];
+};
+
+struct srcu_struct {
+ int completed;
+ struct srcu_struct_array *per_cpu_ref;
+ struct mutex mutex;
+};
+
+#ifndef CONFIG_PREEMPT
+#define srcu_barrier() barrier()
+#else /* #ifndef CONFIG_PREEMPT */
+#define srcu_barrier()
+#endif /* #else #ifndef CONFIG_PREEMPT */
+
+void init_srcu_struct(struct srcu_struct *sp);
+void cleanup_srcu_struct(struct srcu_struct *sp);
+int srcu_read_lock(struct srcu_struct *sp) __acquires(sp);
+void srcu_read_unlock(struct srcu_struct *sp, int idx) __releases(sp);
+void synchronize_srcu(struct srcu_struct *sp);
+long srcu_batches_completed(struct srcu_struct *sp);
+void cleanup_srcu_struct(struct srcu_struct *sp);
diff --git a/kernel/Makefile b/kernel/Makefile
index d948ca12acf0..5e3f3b75563a 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -8,7 +8,7 @@ obj-y = sched.o fork.o exec_domain.o panic.o printk.o profile.o \
signal.o sys.o kmod.o workqueue.o pid.o \
rcupdate.o extable.o params.o posix-timers.o \
kthread.o wait.o kfifo.o sys_ni.o posix-cpu-timers.o mutex.o \
- hrtimer.o rwsem.o latency.o nsproxy.o
+ hrtimer.o rwsem.o latency.o nsproxy.o srcu.o
obj-$(CONFIG_STACKTRACE) += stacktrace.o
obj-y += time/
diff --git a/kernel/srcu.c b/kernel/srcu.c
new file mode 100644
index 000000000000..7e1979f624ba
--- /dev/null
+++ b/kernel/srcu.c
@@ -0,0 +1,257 @@
+/*
+ * Sleepable Read-Copy Update mechanism for mutual exclusion.
+ *
+ * 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, 2006
+ *
+ * Author: Paul McKenney <paulmck@us.ibm.com>
+ *
+ * For detailed explanation of Read-Copy Update mechanism see -
+ * Documentation/RCU/ *.txt
+ *
+ */
+
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/percpu.h>
+#include <linux/preempt.h>
+#include <linux/rcupdate.h>
+#include <linux/sched.h>
+#include <linux/slab.h>
+#include <linux/smp.h>
+#include <linux/srcu.h>
+
+/**
+ * init_srcu_struct - initialize a sleep-RCU structure
+ * @sp: structure to initialize.
+ *
+ * Must invoke this on a given srcu_struct before passing that srcu_struct
+ * to any other function. Each srcu_struct represents a separate domain
+ * of SRCU protection.
+ */
+void init_srcu_struct(struct srcu_struct *sp)
+{
+ sp->completed = 0;
+ sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
+ mutex_init(&sp->mutex);
+}
+
+/*
+ * srcu_readers_active_idx -- returns approximate number of readers
+ * active on the specified rank of per-CPU counters.
+ */
+
+static int srcu_readers_active_idx(struct srcu_struct *sp, int idx)
+{
+ int cpu;
+ int sum;
+
+ sum = 0;
+ for_each_possible_cpu(cpu)
+ sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx];
+ return sum;
+}
+
+/**
+ * srcu_readers_active - returns approximate number of readers.
+ * @sp: which srcu_struct to count active readers (holding srcu_read_lock).
+ *
+ * Note that this is not an atomic primitive, and can therefore suffer
+ * severe errors when invoked on an active srcu_struct. That said, it
+ * can be useful as an error check at cleanup time.
+ */
+int srcu_readers_active(struct srcu_struct *sp)
+{
+ return srcu_readers_active_idx(sp, 0) + srcu_readers_active_idx(sp, 1);
+}
+
+/**
+ * cleanup_srcu_struct - deconstruct a sleep-RCU structure
+ * @sp: structure to clean up.
+ *
+ * Must invoke this after you are finished using a given srcu_struct that
+ * was initialized via init_srcu_struct(), else you leak memory.
+ */
+void cleanup_srcu_struct(struct srcu_struct *sp)
+{
+ int sum;
+
+ sum = srcu_readers_active(sp);
+ WARN_ON(sum); /* Leakage unless caller handles error. */
+ if (sum != 0)
+ return;
+ free_percpu(sp->per_cpu_ref);
+ sp->per_cpu_ref = NULL;
+}
+
+/**
+ * srcu_read_lock - register a new reader for an SRCU-protected structure.
+ * @sp: srcu_struct in which to register the new reader.
+ *
+ * Counts the new reader in the appropriate per-CPU element of the
+ * srcu_struct. Must be called from process context.
+ * Returns an index that must be passed to the matching srcu_read_unlock().
+ */
+int srcu_read_lock(struct srcu_struct *sp)
+{
+ int idx;
+
+ preempt_disable();
+ idx = sp->completed & 0x1;
+ barrier(); /* ensure compiler looks -once- at sp->completed. */
+ per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++;
+ srcu_barrier(); /* ensure compiler won't misorder critical section. */
+ preempt_enable();
+ return idx;
+}
+
+/**
+ * srcu_read_unlock - unregister a old reader from an SRCU-protected structure.
+ * @sp: srcu_struct in which to unregister the old reader.
+ * @idx: return value from corresponding srcu_read_lock().
+ *
+ * Removes the count for the old reader from the appropriate per-CPU
+ * element of the srcu_struct. Note that this may well be a different
+ * CPU than that which was incremented by the corresponding srcu_read_lock().
+ * Must be called from process context.
+ */
+void srcu_read_unlock(struct srcu_struct *sp, int idx)
+{
+ preempt_disable();
+ srcu_barrier(); /* ensure compiler won't misorder critical section. */
+ per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--;
+ preempt_enable();
+}
+
+/**
+ * synchronize_srcu - wait for prior SRCU read-side critical-section completion
+ * @sp: srcu_struct with which to synchronize.
+ *
+ * Flip the completed counter, and wait for the old count to drain to zero.
+ * As with classic RCU, the updater must use some separate means of
+ * synchronizing concurrent updates. Can block; must be called from
+ * process context.
+ *
+ * Note that it is illegal to call synchornize_srcu() from the corresponding
+ * SRCU read-side critical section; doing so will result in deadlock.
+ * However, it is perfectly legal to call synchronize_srcu() on one
+ * srcu_struct from some other srcu_struct's read-side critical section.
+ */
+void synchronize_srcu(struct srcu_struct *sp)
+{
+ int idx;
+
+ idx = sp->completed;
+ mutex_lock(&sp->mutex);
+
+ /*
+ * Check to see if someone else did the work for us while we were
+ * waiting to acquire the lock. We need -two- advances of
+ * the counter, not just one. If there was but one, we might have
+ * shown up -after- our helper's first synchronize_sched(), thus
+ * having failed to prevent CPU-reordering races with concurrent
+ * srcu_read_unlock()s on other CPUs (see comment below). So we
+ * either (1) wait for two or (2) supply the second ourselves.
+ */
+
+ if ((sp->completed - idx) >= 2) {
+ mutex_unlock(&sp->mutex);
+ return;
+ }
+
+ synchronize_sched(); /* Force memory barrier on all CPUs. */
+
+ /*
+ * The preceding synchronize_sched() ensures that any CPU that
+ * sees the new value of sp->completed will also see any preceding
+ * changes to data structures made by this CPU. This prevents
+ * some other CPU from reordering the accesses in its SRCU
+ * read-side critical section to precede the corresponding
+ * srcu_read_lock() -- ensuring that such references will in
+ * fact be protected.
+ *
+ * So it is now safe to do the flip.
+ */
+
+ idx = sp->completed & 0x1;
+ sp->completed++;
+
+ synchronize_sched(); /* Force memory barrier on all CPUs. */
+
+ /*
+ * At this point, because of the preceding synchronize_sched(),
+ * all srcu_read_lock() calls using the old counters have completed.
+ * Their corresponding critical sections might well be still
+ * executing, but the srcu_read_lock() primitives themselves
+ * will have finished executing.
+ */
+
+ while (srcu_readers_active_idx(sp, idx))
+ schedule_timeout_interruptible(1);
+
+ synchronize_sched(); /* Force memory barrier on all CPUs. */
+
+ /*
+ * The preceding synchronize_sched() forces all srcu_read_unlock()
+ * primitives that were executing concurrently with the preceding
+ * for_each_possible_cpu() loop to have completed by this point.
+ * More importantly, it also forces the corresponding SRCU read-side
+ * critical sections to have also completed, and the corresponding
+ * references to SRCU-protected data items to be dropped.
+ *
+ * Note:
+ *
+ * Despite what you might think at first glance, the
+ * preceding synchronize_sched() -must- be within the
+ * critical section ended by the following mutex_unlock().
+ * Otherwise, a task taking the early exit can race
+ * with a srcu_read_unlock(), which might have executed
+ * just before the preceding srcu_readers_active() check,
+ * and whose CPU might have reordered the srcu_read_unlock()
+ * with the preceding critical section. In this case, there
+ * is nothing preventing the synchronize_sched() task that is
+ * taking the early exit from freeing a data structure that
+ * is still being referenced (out of order) by the task
+ * doing the srcu_read_unlock().
+ *
+ * Alternatively, the comparison with "2" on the early exit
+ * could be changed to "3", but this increases synchronize_srcu()
+ * latency for bulk loads. So the current code is preferred.
+ */
+
+ mutex_unlock(&sp->mutex);
+}
+
+/**
+ * srcu_batches_completed - return batches completed.
+ * @sp: srcu_struct on which to report batch completion.
+ *
+ * Report the number of batches, correlated with, but not necessarily
+ * precisely the same as, the number of grace periods that have elapsed.
+ */
+
+long srcu_batches_completed(struct srcu_struct *sp)
+{
+ return sp->completed;
+}
+
+EXPORT_SYMBOL_GPL(init_srcu_struct);
+EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
+EXPORT_SYMBOL_GPL(srcu_read_lock);
+EXPORT_SYMBOL_GPL(srcu_read_unlock);
+EXPORT_SYMBOL_GPL(synchronize_srcu);
+EXPORT_SYMBOL_GPL(srcu_batches_completed);
+EXPORT_SYMBOL_GPL(srcu_readers_active);