[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>

1 files changed, 257 insertions, 0 deletions

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);