[PATCH 3/4] convert rcuref

Nick Piggin
2005-09-24T02:47:15Z

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3/4

--
SUSE Labs, Novell Inc.

[-- Attachment #2: rcu-file-use-atomic-primitives.patch --]
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Use atomic_inc_not_zero for rcu files instead of specal case rcuref.

Signed-off-by: Nick Piggin <npiggin@suse.de>

Index: linux-2.6/include/linux/rcuref.h
===================================================================
--- linux-2.6.orig/include/linux/rcuref.h
+++ /dev/null
@@ -1,220 +0,0 @@
-/*
- * 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_ */
Index: linux-2.6/fs/aio.c
===================================================================
--- linux-2.6.orig/fs/aio.c
+++ linux-2.6/fs/aio.c
@@ -29,7 +29,6 @@
#include <linux/highmem.h>
#include <linux/workqueue.h>
#include <linux/security.h>
-#include <linux/rcuref.h>

#include <asm/kmap_types.h>
#include <asm/uaccess.h>
@@ -500,7 +499,7 @@ static int __aio_put_req(struct kioctx *
/* Must be done under the lock to serialise against cancellation.
* Call this aio_fput as it duplicates fput via the fput_work.
*/
- if (unlikely(rcuref_dec_and_test(&req->ki_filp->f_count))) {
+ if (unlikely(atomic_dec_and_test(&req->ki_filp->f_count))) {
get_ioctx(ctx);
spin_lock(&fput_lock);
list_add(&req->ki_list, &fput_head);
Index: linux-2.6/fs/file_table.c
===================================================================
--- linux-2.6.orig/fs/file_table.c
+++ linux-2.6/fs/file_table.c
@@ -117,7 +117,7 @@ EXPORT_SYMBOL(get_empty_filp);

void fastcall fput(struct file *file)
{
- if (rcuref_dec_and_test(&file->f_count))
+ if (atomic_dec_and_test(&file->f_count))
__fput(file);
}

@@ -166,7 +166,7 @@ struct file fastcall *fget(unsigned int
rcu_read_lock();
file = fcheck_files(files, fd);
if (file) {
- if (!rcuref_inc_lf(&file->f_count)) {
+ if (!atomic_inc_not_zero(&file->f_count)) {
/* File object ref couldn't be taken */
rcu_read_unlock();
return NULL;
@@ -198,7 +198,7 @@ struct file fastcall *fget_light(unsigne
rcu_read_lock();
file = fcheck_files(files, fd);
if (file) {
- if (rcuref_inc_lf(&file->f_count))
+ if (atomic_inc_not_zero(&file->f_count))
*fput_needed = 1;
else
/* Didn't get the reference, someone's freed */
@@ -213,7 +213,7 @@ struct file fastcall *fget_light(unsigne

void put_filp(struct file *file)
{
- if (rcuref_dec_and_test(&file->f_count)) {
+ if (atomic_dec_and_test(&file->f_count)) {
security_file_free(file);
file_kill(file);
file_free(file);
Index: linux-2.6/include/linux/fs.h
===================================================================
--- linux-2.6.orig/include/linux/fs.h
+++ linux-2.6/include/linux/fs.h
@@ -9,7 +9,6 @@
#include <linux/config.h>
#include <linux/limits.h>
#include <linux/ioctl.h>
-#include <linux/rcuref.h>

/*
* It's silly to have NR_OPEN bigger than NR_FILE, but you can change
@@ -604,7 +603,7 @@ extern spinlock_t files_lock;
#define file_list_lock() spin_lock(&files_lock);
#define file_list_unlock() spin_unlock(&files_lock);

-#define get_file(x) rcuref_inc(&(x)->f_count)
+#define get_file(x) atomic_inc(&(x)->f_count)
#define file_count(x) atomic_read(&(x)->f_count)

#define MAX_NON_LFS ((1UL<<31) - 1)
Index: linux-2.6/Documentation/RCU/rcuref.txt
===================================================================
--- linux-2.6.orig/Documentation/RCU/rcuref.txt
+++ linux-2.6/Documentation/RCU/rcuref.txt
@@ -1,74 +1,67 @@
-Refcounter framework for elements of lists/arrays protected by
-RCU.
+Refcounter design for elements of lists/arrays protected by RCU.

Refcounting on elements of lists which are protected by traditional
reader/writer spinlocks or semaphores are straight forward as in:

-1. 2.
-add() search_and_reference()
-{ {
- alloc_object read_lock(&list_lock);
- ... search_for_element
- atomic_set(&el->rc, 1); atomic_inc(&el->rc);
- write_lock(&list_lock); ...
- add_element read_unlock(&list_lock);
- ... ...
- write_unlock(&list_lock); }
+1. 2.
+add() search_and_reference()
+{ {
+ alloc_object read_lock(&list_lock);
+ ... search_for_element
+ atomic_set(&el->rc, 1); atomic_inc(&el->rc);
+ write_lock(&list_lock); ...
+ add_element read_unlock(&list_lock);
+ ... ...
+ write_unlock(&list_lock); }
}

3. 4.
release_referenced() delete()
{ {
- ... write_lock(&list_lock);
- atomic_dec(&el->rc, relfunc) ...
- ... delete_element
-} write_unlock(&list_lock);
- ...
- if (atomic_dec_and_test(&el->rc))
- kfree(el);
- ...
+ ... write_lock(&list_lock);
+ atomic_dec(&el->rc, relfunc) ...
+ ... delete_element
+} write_unlock(&list_lock);
+ ...
+ if (atomic_dec_and_test(&el->rc))
+ kfree(el);
+ ...
}

If this list/array is made lock free using rcu as in changing the
write_lock in add() and delete() to spin_lock and changing read_lock
-in search_and_reference to rcu_read_lock(), the rcuref_get in
+in search_and_reference to rcu_read_lock(), the atomic_get in
search_and_reference could potentially hold reference to an element which
-has already been deleted from the list/array. rcuref_lf_get_rcu takes
+has already been deleted from the list/array. atomic_inc_not_zero takes
care of this scenario. search_and_reference should look as;

1. 2.
add() search_and_reference()
{ {
- alloc_object rcu_read_lock();
- ... search_for_element
- atomic_set(&el->rc, 1); if (rcuref_inc_lf(&el->rc)) {
- write_lock(&list_lock); rcu_read_unlock();
- return FAIL;
- add_element }
- ... ...
- write_unlock(&list_lock); rcu_read_unlock();
+ alloc_object rcu_read_lock();
+ ... search_for_element
+ atomic_set(&el->rc, 1); if (atomic_inc_not_zero(&el->rc)) {
+ write_lock(&list_lock); rcu_read_unlock();
+ return FAIL;
+ add_element }
+ ... ...
+ write_unlock(&list_lock); rcu_read_unlock();
} }
3. 4.
release_referenced() delete()
{ {
- ... write_lock(&list_lock);
- rcuref_dec(&el->rc, relfunc) ...
- ... delete_element
-} write_unlock(&list_lock);
- ...
- if (rcuref_dec_and_test(&el->rc))
- call_rcu(&el->head, el_free);
- ...
+ ... write_lock(&list_lock);
+ atomic_dec(&el->rc, relfunc) ...
+ ... delete_element
+} write_unlock(&list_lock);
+ ...
+ if (atomic_dec_and_test(&el->rc))
+ call_rcu(&el->head, el_free);
+ ...
}

Sometimes, reference to the element need to be obtained in the
-update (write) stream. In such cases, rcuref_inc_lf might be an overkill
-since the spinlock serialising list updates are held. rcuref_inc
+update (write) stream. In such cases, atomic_inc_not_zero might be an
+overkill since the spinlock serialising list updates are held. atomic_inc
is to be used in such cases.
-For arches which do not have cmpxchg rcuref_inc_lf
-api uses a hashed spinlock implementation and the same hashed spinlock
-is acquired in all rcuref_xxx primitives to preserve atomicity.
-Note: Use rcuref_inc api only if you need to use rcuref_inc_lf on the
-refcounter atleast at one place. Mixing rcuref_inc and atomic_xxx api
-might lead to races. rcuref_inc_lf() must be used in lockfree
-RCU critical sections only.
+
Index: linux-2.6/kernel/rcupdate.c
===================================================================
--- linux-2.6.orig/kernel/rcupdate.c
+++ linux-2.6/kernel/rcupdate.c
@@ -45,7 +45,6 @@
#include <linux/percpu.h>
#include <linux/notifier.h>
#include <linux/rcupdate.h>
-#include <linux/rcuref.h>
#include <linux/cpu.h>

/* Definition for rcupdate control block. */
@@ -73,19 +72,6 @@ DEFINE_PER_CPU(struct rcu_data, rcu_bh_d
static DEFINE_PER_CPU(struct tasklet_struct, rcu_tasklet) = {NULL};
static int maxbatch = 10;

-#ifndef __HAVE_ARCH_CMPXCHG
-/*
- * We use an array of spinlocks for the rcurefs -- similar to ones in sparc
- * 32 bit atomic_t implementations, and a hash function similar to that
- * for our refcounting needs.
- * Can't help multiprocessors which donot have cmpxchg :(
- */
-
-spinlock_t __rcuref_hash[RCUREF_HASH_SIZE] = {
- [0 ... (RCUREF_HASH_SIZE-1)] = SPIN_LOCK_UNLOCKED
-};
-#endif
-
/**
* call_rcu - Queue an RCU callback for invocation after a grace period.
* @head: structure to be used for queueing the RCU updates.