Documentation / RCU / rcuref.txt

Based on kernel version 2.6.25. Page generated on 2008-04-18 21:22 EST.

	Reference-count design for elements of lists/arrays protected by RCU.
	
	Reference counting on elements of lists which are protected by traditional
	reader/writer spinlocks or semaphores are straightforward:
	
	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 atomic_get in
	search_and_reference could potentially hold reference to an element which
	has already been deleted from the list/array.  Use atomic_inc_not_zero()
	in this scenario as follows:
	
	1.					2.
	add()					search_and_reference()
	{					{
	    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);
	    if (atomic_dec_and_test(&el->rc))       ...
	        call_rcu(&el->head, el_free);       delete_element
	    ...                                     write_unlock(&list_lock);
	} 					    ...
						    if (atomic_dec_and_test(&el->rc))
						        call_rcu(&el->head, el_free);
						    ...
						}
	
	Sometimes, a reference to the element needs to be obtained in the
	update (write) stream.  In such cases, atomic_inc_not_zero() might be
	overkill, since we hold the update-side spinlock.  One might instead
	use atomic_inc() in such cases.

• whatisRCU.txt
• rcuref.txt
• checklist.txt
• torture.txt
• listRCU.txt
• UP.txt
• arrayRCU.txt
• RTFP.txt
• rcu.txt
• [ RCU ]
• NMI-RCU.txt
• 00-INDEX
•