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[PATCH] files: rcuref APIs

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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>
This commit is contained in:
Dipankar Sarma 2005-09-09 13:04:09 -07:00 committed by Linus Torvalds
parent 8b6490e5fa
commit c0dfb29051
3 changed files with 308 additions and 0 deletions
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74
Documentation/RCU/rcuref.txt Normal file
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@ -0,0 +1,74 @@
Refcounter framework 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); }
}
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);
...
}
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
search_and_reference could potentially hold reference to an element which
has already been deleted from the list/array. rcuref_lf_get_rcu 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();
} }
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);
...
}
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
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.

220
include/linux/rcuref.h Normal file
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@ -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_ */

14
kernel/rcupdate.c
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@ -45,6 +45,7 @@
#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. */
@ -72,6 +73,19 @@ DEFINE_PER_CPU(struct rcu_data, rcu_bh_data) = { 0L };
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.