[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>
This commit is contained in:
Dipankar Sarma
Linus Torvalds
parent
8b6490e5fa
commit
c0dfb29051
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Refcounter framework for elements of lists/arrays protected by
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RCU.
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Refcounting on elements of lists which are protected by traditional
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reader/writer spinlocks or semaphores are straight forward as in:
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1. 2.
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add() search_and_reference()
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{ {
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alloc_object read_lock(&list_lock);
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... search_for_element
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atomic_set(&el->rc, 1); atomic_inc(&el->rc);
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write_lock(&list_lock); ...
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add_element read_unlock(&list_lock);
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... ...
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write_unlock(&list_lock); }
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}
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3. 4.
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release_referenced() delete()
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{ {
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... write_lock(&list_lock);
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atomic_dec(&el->rc, relfunc) ...
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... delete_element
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} write_unlock(&list_lock);
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...
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if (atomic_dec_and_test(&el->rc))
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kfree(el);
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...
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}
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If this list/array is made lock free using rcu as in changing the
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write_lock in add() and delete() to spin_lock and changing read_lock
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in search_and_reference to rcu_read_lock(), the rcuref_get in
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search_and_reference could potentially hold reference to an element which
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has already been deleted from the list/array. rcuref_lf_get_rcu takes
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care of this scenario. search_and_reference should look as;
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1. 2.
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add() search_and_reference()
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{ {
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alloc_object rcu_read_lock();
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... search_for_element
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atomic_set(&el->rc, 1); if (rcuref_inc_lf(&el->rc)) {
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write_lock(&list_lock); rcu_read_unlock();
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return FAIL;
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add_element }
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... ...
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write_unlock(&list_lock); rcu_read_unlock();
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} }
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3. 4.
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release_referenced() delete()
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{ {
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... write_lock(&list_lock);
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rcuref_dec(&el->rc, relfunc) ...
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... delete_element
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} write_unlock(&list_lock);
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...
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if (rcuref_dec_and_test(&el->rc))
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call_rcu(&el->head, el_free);
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...
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}
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Sometimes, reference to the element need to be obtained in the
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update (write) stream. In such cases, rcuref_inc_lf might be an overkill
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since the spinlock serialising list updates are held. rcuref_inc
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is to be used in such cases.
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For arches which do not have cmpxchg rcuref_inc_lf
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api uses a hashed spinlock implementation and the same hashed spinlock
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is acquired in all rcuref_xxx primitives to preserve atomicity.
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Note: Use rcuref_inc api only if you need to use rcuref_inc_lf on the
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refcounter atleast at one place. Mixing rcuref_inc and atomic_xxx api
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might lead to races. rcuref_inc_lf() must be used in lockfree
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RCU critical sections only.
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@ -0,0 +1,220 @@
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/*
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* rcuref.h
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*
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* Reference counting for elements of lists/arrays protected by
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* RCU.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
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*
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* Copyright (C) IBM Corporation, 2005
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*
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* Author: Dipankar Sarma <dipankar@in.ibm.com>
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* Ravikiran Thirumalai <kiran_th@gmail.com>
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*
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* See Documentation/RCU/rcuref.txt for detailed user guide.
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*
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*/
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#ifndef _RCUREF_H_
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#define _RCUREF_H_
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#ifdef __KERNEL__
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#include <linux/types.h>
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#include <linux/interrupt.h>
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#include <linux/spinlock.h>
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#include <asm/atomic.h>
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/*
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* These APIs work on traditional atomic_t counters used in the
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* kernel for reference counting. Under special circumstances
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* where a lock-free get() operation races with a put() operation
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* these APIs can be used. See Documentation/RCU/rcuref.txt.
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*/
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#ifdef __HAVE_ARCH_CMPXCHG
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/**
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* rcuref_inc - increment refcount for object.
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* @rcuref: reference counter in the object in question.
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*
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* This should be used only for objects where we use RCU and
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* use the rcuref_inc_lf() api to acquire a reference
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* in a lock-free reader-side critical section.
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*/
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static inline void rcuref_inc(atomic_t *rcuref)
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{
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atomic_inc(rcuref);
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}
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/**
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* rcuref_dec - decrement refcount for object.
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* @rcuref: reference counter in the object in question.
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*
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* This should be used only for objects where we use RCU and
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* use the rcuref_inc_lf() api to acquire a reference
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* in a lock-free reader-side critical section.
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*/
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static inline void rcuref_dec(atomic_t *rcuref)
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{
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atomic_dec(rcuref);
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}
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/**
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* rcuref_dec_and_test - decrement refcount for object and test
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* @rcuref: reference counter in the object.
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* @release: pointer to the function that will clean up the object
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* when the last reference to the object is released.
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* This pointer is required.
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*
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* Decrement the refcount, and if 0, return 1. Else return 0.
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*
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* This should be used only for objects where we use RCU and
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* use the rcuref_inc_lf() api to acquire a reference
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* in a lock-free reader-side critical section.
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*/
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static inline int rcuref_dec_and_test(atomic_t *rcuref)
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{
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return atomic_dec_and_test(rcuref);
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}
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/*
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* cmpxchg is needed on UP too, if deletions to the list/array can happen
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* in interrupt context.
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*/
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/**
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* rcuref_inc_lf - Take reference to an object in a read-side
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* critical section protected by RCU.
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* @rcuref: reference counter in the object in question.
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*
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* Try and increment the refcount by 1. The increment might fail if
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* the reference counter has been through a 1 to 0 transition and
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* is no longer part of the lock-free list.
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* Returns non-zero on successful increment and zero otherwise.
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*/
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static inline int rcuref_inc_lf(atomic_t *rcuref)
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{
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int c, old;
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c = atomic_read(rcuref);
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while (c && (old = cmpxchg(&rcuref->counter, c, c + 1)) != c)
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c = old;
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return c;
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}
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#else /* !__HAVE_ARCH_CMPXCHG */
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extern spinlock_t __rcuref_hash[];
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/*
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* Use a hash table of locks to protect the reference count
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* since cmpxchg is not available in this arch.
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*/
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#ifdef CONFIG_SMP
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#define RCUREF_HASH_SIZE 4
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#define RCUREF_HASH(k) \
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(&__rcuref_hash[(((unsigned long)k)>>8) & (RCUREF_HASH_SIZE-1)])
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#else
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#define RCUREF_HASH_SIZE 1
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#define RCUREF_HASH(k) &__rcuref_hash[0]
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#endif /* CONFIG_SMP */
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/**
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* rcuref_inc - increment refcount for object.
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* @rcuref: reference counter in the object in question.
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*
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* This should be used only for objects where we use RCU and
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* use the rcuref_inc_lf() api to acquire a reference in a lock-free
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* reader-side critical section.
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*/
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static inline void rcuref_inc(atomic_t *rcuref)
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{
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unsigned long flags;
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spin_lock_irqsave(RCUREF_HASH(rcuref), flags);
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rcuref->counter += 1;
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spin_unlock_irqrestore(RCUREF_HASH(rcuref), flags);
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}
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/**
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* rcuref_dec - decrement refcount for object.
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* @rcuref: reference counter in the object in question.
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*
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* This should be used only for objects where we use RCU and
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* use the rcuref_inc_lf() api to acquire a reference in a lock-free
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* reader-side critical section.
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*/
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static inline void rcuref_dec(atomic_t *rcuref)
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{
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unsigned long flags;
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spin_lock_irqsave(RCUREF_HASH(rcuref), flags);
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rcuref->counter -= 1;
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spin_unlock_irqrestore(RCUREF_HASH(rcuref), flags);
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}
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/**
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* rcuref_dec_and_test - decrement refcount for object and test
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* @rcuref: reference counter in the object.
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* @release: pointer to the function that will clean up the object
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* when the last reference to the object is released.
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* This pointer is required.
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*
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* Decrement the refcount, and if 0, return 1. Else return 0.
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*
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* This should be used only for objects where we use RCU and
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* use the rcuref_inc_lf() api to acquire a reference in a lock-free
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* reader-side critical section.
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*/
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static inline int rcuref_dec_and_test(atomic_t *rcuref)
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{
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unsigned long flags;
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spin_lock_irqsave(RCUREF_HASH(rcuref), flags);
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rcuref->counter--;
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if (!rcuref->counter) {
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spin_unlock_irqrestore(RCUREF_HASH(rcuref), flags);
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return 1;
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} else {
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spin_unlock_irqrestore(RCUREF_HASH(rcuref), flags);
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return 0;
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}
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}
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/**
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* rcuref_inc_lf - Take reference to an object of a lock-free collection
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* by traversing a lock-free list/array.
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* @rcuref: reference counter in the object in question.
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*
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* Try and increment the refcount by 1. The increment might fail if
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* the reference counter has been through a 1 to 0 transition and
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* object is no longer part of the lock-free list.
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* Returns non-zero on successful increment and zero otherwise.
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*/
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static inline int rcuref_inc_lf(atomic_t *rcuref)
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{
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int ret;
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unsigned long flags;
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spin_lock_irqsave(RCUREF_HASH(rcuref), flags);
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if (rcuref->counter)
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ret = rcuref->counter++;
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else
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ret = 0;
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spin_unlock_irqrestore(RCUREF_HASH(rcuref), flags);
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return ret;
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}
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#endif /* !__HAVE_ARCH_CMPXCHG */
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#endif /* __KERNEL__ */
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#endif /* _RCUREF_H_ */
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@ -45,6 +45,7 @@
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#include <linux/percpu.h>
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#include <linux/notifier.h>
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#include <linux/rcupdate.h>
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#include <linux/rcuref.h>
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#include <linux/cpu.h>
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/* Definition for rcupdate control block. */
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@ -72,6 +73,19 @@ DEFINE_PER_CPU(struct rcu_data, rcu_bh_data) = { 0L };
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static DEFINE_PER_CPU(struct tasklet_struct, rcu_tasklet) = {NULL};
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static int maxbatch = 10;
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#ifndef __HAVE_ARCH_CMPXCHG
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/*
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* We use an array of spinlocks for the rcurefs -- similar to ones in sparc
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* 32 bit atomic_t implementations, and a hash function similar to that
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* for our refcounting needs.
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* Can't help multiprocessors which donot have cmpxchg :(
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*/
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spinlock_t __rcuref_hash[RCUREF_HASH_SIZE] = {
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[0 ... (RCUREF_HASH_SIZE-1)] = SPIN_LOCK_UNLOCKED
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};
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#endif
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/**
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* call_rcu - Queue an RCU callback for invocation after a grace period.
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* @head: structure to be used for queueing the RCU updates.
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