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d19720a909
Updates to in-tree RCU documentation based on comments over the past few months. Signed-off-by: "Paul E. McKenney" <paulmck@us.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
67 lines
2.2 KiB
Plaintext
67 lines
2.2 KiB
Plaintext
Reference-count design for elements of lists/arrays protected by RCU.
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Reference counting on elements of lists which are protected by traditional
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reader/writer spinlocks or semaphores are straightforward:
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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 atomic_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. Use atomic_inc_not_zero()
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in this scenario as follows:
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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 (atomic_inc_not_zero(&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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if (atomic_dec_and_test(&el->rc)) ...
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call_rcu(&el->head, el_free); 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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call_rcu(&el->head, el_free);
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...
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}
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Sometimes, a reference to the element needs to be obtained in the
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update (write) stream. In such cases, atomic_inc_not_zero() might be
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overkill, since we hold the update-side spinlock. One might instead
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use atomic_inc() in such cases.
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