[PATCH] files: rcuref APIs

Adds a set of primitives to do reference counting for objects that are looked
up without locks using RCU.

Signed-off-by: Ravikiran Thirumalai <kiran_th@gmail.com>
Signed-off-by: Dipankar Sarma <dipankar@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 220 insertions, 0 deletions

diff --git a/include/linux/rcuref.h b/include/linux/rcuref.h
new file mode 100644
index 00000000000..e1adbba14b6
--- /dev/null
+++ b/include/linux/rcuref.h
@@ -0,0 +1,220 @@
+/*
+ * rcuref.h
+ *
+ * Reference counting for elements of lists/arrays protected by
+ * RCU.
+ *
+ * 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
+ *
+ * Author: Dipankar Sarma <dipankar@in.ibm.com>
+ *	   Ravikiran Thirumalai <kiran_th@gmail.com>
+ *
+ * See Documentation/RCU/rcuref.txt for detailed user guide.
+ *
+ */
+
+#ifndef _RCUREF_H_
+#define _RCUREF_H_
+
+#ifdef __KERNEL__
+
+#include <linux/types.h>
+#include <linux/interrupt.h>
+#include <linux/spinlock.h>
+#include <asm/atomic.h>
+
+/*
+ * These APIs work on traditional atomic_t counters used in the
+ * kernel for reference counting. Under special circumstances
+ * where a lock-free get() operation races with a put() operation
+ * these APIs can be used. See Documentation/RCU/rcuref.txt.
+ */
+
+#ifdef __HAVE_ARCH_CMPXCHG
+
+/**
+ * rcuref_inc - increment refcount for object.
+ * @rcuref: reference counter in the object in question.
+ *
+ * This should be used only for objects where we use RCU and
+ * use the rcuref_inc_lf() api to acquire a reference
+ * in a lock-free reader-side critical section.
+ */
+static inline void rcuref_inc(atomic_t *rcuref)
+{
+	atomic_inc(rcuref);
+}
+
+/**
+ * rcuref_dec - decrement refcount for object.
+ * @rcuref: reference counter in the object in question.
+ *
+ * This should be used only for objects where we use RCU and
+ * use the rcuref_inc_lf() api to acquire a reference
+ * in a lock-free reader-side critical section.
+ */
+static inline void rcuref_dec(atomic_t *rcuref)
+{
+	atomic_dec(rcuref);
+}
+
+/**
+ * rcuref_dec_and_test - decrement refcount for object and test
+ * @rcuref: reference counter in the object.
+ * @release: pointer to the function that will clean up the object
+ *	     when the last reference to the object is released.
+ *	     This pointer is required.
+ *
+ * Decrement the refcount, and if 0, return 1. Else return 0.
+ *
+ * This should be used only for objects where we use RCU and
+ * use the rcuref_inc_lf() api to acquire a reference
+ * in a lock-free reader-side critical section.
+ */
+static inline int rcuref_dec_and_test(atomic_t *rcuref)
+{
+	return atomic_dec_and_test(rcuref);
+}
+
+/*
+ * cmpxchg is needed on UP too, if deletions to the list/array can happen
+ * in interrupt context.
+ */
+
+/**
+ * rcuref_inc_lf - Take reference to an object in a read-side
+ * critical section protected by RCU.
+ * @rcuref: reference counter in the object in question.
+ *
+ * Try and increment the refcount by 1.  The increment might fail if
+ * the reference counter has been through a 1 to 0 transition and
+ * is no longer part of the lock-free list.
+ * Returns non-zero on successful increment and zero otherwise.
+ */
+static inline int rcuref_inc_lf(atomic_t *rcuref)
+{
+	int c, old;
+	c = atomic_read(rcuref);
+	while (c && (old = cmpxchg(&rcuref->counter, c, c + 1)) != c)
+		c = old;
+	return c;
+}
+
+#else				/* !__HAVE_ARCH_CMPXCHG */
+
+extern spinlock_t __rcuref_hash[];
+
+/*
+ * Use a hash table of locks to protect the reference count
+ * since cmpxchg is not available in this arch.
+ */
+#ifdef	CONFIG_SMP
+#define RCUREF_HASH_SIZE	4
+#define RCUREF_HASH(k) \
+	(&__rcuref_hash[(((unsigned long)k)>>8) & (RCUREF_HASH_SIZE-1)])
+#else
+#define	RCUREF_HASH_SIZE	1
+#define RCUREF_HASH(k) 	&__rcuref_hash[0]
+#endif				/* CONFIG_SMP */
+
+/**
+ * rcuref_inc - increment refcount for object.
+ * @rcuref: reference counter in the object in question.
+ *
+ * This should be used only for objects where we use RCU and
+ * use the rcuref_inc_lf() api to acquire a reference in a lock-free
+ * reader-side critical section.
+ */
+static inline void rcuref_inc(atomic_t *rcuref)
+{
+	unsigned long flags;
+	spin_lock_irqsave(RCUREF_HASH(rcuref), flags);
+	rcuref->counter += 1;
+	spin_unlock_irqrestore(RCUREF_HASH(rcuref), flags);
+}
+
+/**
+ * rcuref_dec - decrement refcount for object.
+ * @rcuref: reference counter in the object in question.
+ *
+ * This should be used only for objects where we use RCU and
+ * use the rcuref_inc_lf() api to acquire a reference in a lock-free
+ * reader-side critical section.
+ */
+static inline void rcuref_dec(atomic_t *rcuref)
+{
+	unsigned long flags;
+	spin_lock_irqsave(RCUREF_HASH(rcuref), flags);
+	rcuref->counter -= 1;
+	spin_unlock_irqrestore(RCUREF_HASH(rcuref), flags);
+}
+
+/**
+ * rcuref_dec_and_test - decrement refcount for object and test
+ * @rcuref: reference counter in the object.
+ * @release: pointer to the function that will clean up the object
+ *	     when the last reference to the object is released.
+ *	     This pointer is required.
+ *
+ * Decrement the refcount, and if 0, return 1. Else return 0.
+ *
+ * This should be used only for objects where we use RCU and
+ * use the rcuref_inc_lf() api to acquire a reference in a lock-free
+ * reader-side critical section.
+ */
+static inline int rcuref_dec_and_test(atomic_t *rcuref)
+{
+	unsigned long flags;
+	spin_lock_irqsave(RCUREF_HASH(rcuref), flags);
+	rcuref->counter--;
+	if (!rcuref->counter) {
+		spin_unlock_irqrestore(RCUREF_HASH(rcuref), flags);
+		return 1;
+	} else {
+		spin_unlock_irqrestore(RCUREF_HASH(rcuref), flags);
+		return 0;
+	}
+}
+
+/**
+ * rcuref_inc_lf - Take reference to an object of a lock-free collection
+ * by traversing a lock-free list/array.
+ * @rcuref: reference counter in the object in question.
+ *
+ * Try and increment the refcount by 1.  The increment might fail if
+ * the reference counter has been through a 1 to 0 transition and
+ * object is no longer part of the lock-free list.
+ * Returns non-zero on successful increment and zero otherwise.
+ */
+static inline int rcuref_inc_lf(atomic_t *rcuref)
+{
+	int ret;
+	unsigned long flags;
+	spin_lock_irqsave(RCUREF_HASH(rcuref), flags);
+	if (rcuref->counter)
+		ret = rcuref->counter++;
+	else
+		ret = 0;
+	spin_unlock_irqrestore(RCUREF_HASH(rcuref), flags);
+	return ret;
+}
+
+
+#endif /* !__HAVE_ARCH_CMPXCHG */
+
+#endif /* __KERNEL__ */
+#endif /* _RCUREF_H_ */