About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / RCU / rcuref.txt




Custom Search

Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.

1	Reference-count design for elements of lists/arrays protected by RCU.
2	
3	Reference counting on elements of lists which are protected by traditional
4	reader/writer spinlocks or semaphores are straightforward:
5	
6	1.				2.
7	add()				search_and_reference()
8	{				{
9	    alloc_object		    read_lock(&list_lock);
10	    ...				    search_for_element
11	    atomic_set(&el->rc, 1);	    atomic_inc(&el->rc);
12	    write_lock(&list_lock);	     ...
13	    add_element			    read_unlock(&list_lock);
14	    ...				    ...
15	    write_unlock(&list_lock);	}
16	}
17	
18	3.					4.
19	release_referenced()			delete()
20	{					{
21	    ...					    write_lock(&list_lock);
22	    atomic_dec(&el->rc, relfunc)	    ...
23	    ...					    remove_element
24	}					    write_unlock(&list_lock);
25	 					    ...
26						    if (atomic_dec_and_test(&el->rc))
27						        kfree(el);
28						    ...
29						}
30	
31	If this list/array is made lock free using RCU as in changing the
32	write_lock() in add() and delete() to spin_lock() and changing read_lock()
33	in search_and_reference() to rcu_read_lock(), the atomic_inc() in
34	search_and_reference() could potentially hold reference to an element which
35	has already been deleted from the list/array.  Use atomic_inc_not_zero()
36	in this scenario as follows:
37	
38	1.					2.
39	add()					search_and_reference()
40	{					{
41	    alloc_object			    rcu_read_lock();
42	    ...					    search_for_element
43	    atomic_set(&el->rc, 1);		    if (!atomic_inc_not_zero(&el->rc)) {
44	    spin_lock(&list_lock);		        rcu_read_unlock();
45						        return FAIL;
46	    add_element				    }
47	    ...					    ...
48	    spin_unlock(&list_lock);		    rcu_read_unlock();
49	}					}
50	3.					4.
51	release_referenced()			delete()
52	{					{
53	    ...					    spin_lock(&list_lock);
54	    if (atomic_dec_and_test(&el->rc))       ...
55	        call_rcu(&el->head, el_free);       remove_element
56	    ...                                     spin_unlock(&list_lock);
57	} 					    ...
58						    if (atomic_dec_and_test(&el->rc))
59						        call_rcu(&el->head, el_free);
60						    ...
61						}
62	
63	Sometimes, a reference to the element needs to be obtained in the
64	update (write) stream.  In such cases, atomic_inc_not_zero() might be
65	overkill, since we hold the update-side spinlock.  One might instead
66	use atomic_inc() in such cases.
67	
68	It is not always convenient to deal with "FAIL" in the
69	search_and_reference() code path.  In such cases, the
70	atomic_dec_and_test() may be moved from delete() to el_free()
71	as follows:
72	
73	1.					2.
74	add()					search_and_reference()
75	{					{
76	    alloc_object			    rcu_read_lock();
77	    ...					    search_for_element
78	    atomic_set(&el->rc, 1);		    atomic_inc(&el->rc);
79	    spin_lock(&list_lock);		    ...
80	
81	    add_element				    rcu_read_unlock();
82	    ...					}
83	    spin_unlock(&list_lock);		4.
84	}					delete()
85	3.					{
86	release_referenced()			    spin_lock(&list_lock);
87	{					    ...
88	    ...					    remove_element
89	    if (atomic_dec_and_test(&el->rc))       spin_unlock(&list_lock);
90	        kfree(el);			    ...
91	    ...                                     call_rcu(&el->head, el_free);
92	} 					    ...
93	5.					}
94	void el_free(struct rcu_head *rhp)
95	{
96	    release_referenced();
97	}
98	
99	The key point is that the initial reference added by add() is not removed
100	until after a grace period has elapsed following removal.  This means that
101	search_and_reference() cannot find this element, which means that the value
102	of el->rc cannot increase.  Thus, once it reaches zero, there are no
103	readers that can or ever will be able to reference the element.  The
104	element can therefore safely be freed.  This in turn guarantees that if
105	any reader finds the element, that reader may safely acquire a reference
106	without checking the value of the reference counter.
107	
108	In cases where delete() can sleep, synchronize_rcu() can be called from
109	delete(), so that el_free() can be subsumed into delete as follows:
110	
111	4.
112	delete()
113	{
114	    spin_lock(&list_lock);
115	    ...
116	    remove_element
117	    spin_unlock(&list_lock);
118	    ...
119	    synchronize_rcu();
120	    if (atomic_dec_and_test(&el->rc))
121	    	kfree(el);
122	    ...
123	}
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.