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c0dfb29051
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>
75 lines
2.3 KiB
Plaintext
75 lines
2.3 KiB
Plaintext
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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