1This document contains brief definitions of LKMM-related terms.  Like most
2glossaries, it is not intended to be read front to back (except perhaps
3as a way of confirming a diagnosis of OCD), but rather to be searched
4for specific terms.
5
6
7Address Dependency:  When the address of a later memory access is computed
8	based on the value returned by an earlier load, an "address
9	dependency" extends from that load extending to the later access.
10	Address dependencies are quite common in RCU read-side critical
11	sections:
12
13	 1 rcu_read_lock();
14	 2 p = rcu_dereference(gp);
15	 3 do_something(p->a);
16	 4 rcu_read_unlock();
17
18	 In this case, because the address of "p->a" on line 3 is computed
19	 from the value returned by the rcu_dereference() on line 2, the
20	 address dependency extends from that rcu_dereference() to that
21	 "p->a".  In rare cases, optimizing compilers can destroy address
22	 dependencies.	Please see Documentation/RCU/rcu_dereference.txt
23	 for more information.
24
25	 See also "Control Dependency" and "Data Dependency".
26
27Acquire:  With respect to a lock, acquiring that lock, for example,
28	using spin_lock().  With respect to a non-lock shared variable,
29	a special operation that includes a load and which orders that
30	load before later memory references running on that same CPU.
31	An example special acquire operation is smp_load_acquire(),
32	but atomic_read_acquire() and atomic_xchg_acquire() also include
33	acquire loads.
34
35	When an acquire load returns the value stored by a release store
36	to that same variable, (in other words, the acquire load "reads
37	from" the release store), then all operations preceding that
38	store "happen before" any operations following that load acquire.
39
40	See also "Happens-Before", "Reads-From", "Relaxed", and "Release".
41
42Coherence (co):  When one CPU's store to a given variable overwrites
43	either the value from another CPU's store or some later value,
44	there is said to be a coherence link from the second CPU to
45	the first.
46
47	It is also possible to have a coherence link within a CPU, which
48	is a "coherence internal" (coi) link.  The term "coherence
49	external" (coe) link is used when it is necessary to exclude
50	the coi case.
51
52	See also "From-reads" and "Reads-from".
53
54Control Dependency:  When a later store's execution depends on a test
55	of a value computed from a value returned by an earlier load,
56	a "control dependency" extends from that load to that store.
57	For example:
58
59	 1 if (READ_ONCE(x))
60	 2   WRITE_ONCE(y, 1);
61
62	 Here, the control dependency extends from the READ_ONCE() on
63	 line 1 to the WRITE_ONCE() on line 2.	Control dependencies are
64	 fragile, and can be easily destroyed by optimizing compilers.
65	 Please see control-dependencies.txt for more information.
66
67	 See also "Address Dependency" and "Data Dependency".
68
69Cycle:	Memory-barrier pairing is restricted to a pair of CPUs, as the
70	name suggests.	And in a great many cases, a pair of CPUs is all
71	that is required.  In other cases, the notion of pairing must be
72	extended to additional CPUs, and the result is called a "cycle".
73	In a cycle, each CPU's ordering interacts with that of the next:
74
75	CPU 0                CPU 1                CPU 2
76	WRITE_ONCE(x, 1);    WRITE_ONCE(y, 1);    WRITE_ONCE(z, 1);
77	smp_mb();            smp_mb();            smp_mb();
78	r0 = READ_ONCE(y);   r1 = READ_ONCE(z);   r2 = READ_ONCE(x);
79
80	CPU 0's smp_mb() interacts with that of CPU 1, which interacts
81	with that of CPU 2, which in turn interacts with that of CPU 0
82	to complete the cycle.	Because of the smp_mb() calls between
83	each pair of memory accesses, the outcome where r0, r1, and r2
84	are all equal to zero is forbidden by LKMM.
85
86	See also "Pairing".
87
88Data Dependency:  When the data written by a later store is computed based
89	on the value returned by an earlier load, a "data dependency"
90	extends from that load to that later store.  For example:
91
92	 1 r1 = READ_ONCE(x);
93	 2 WRITE_ONCE(y, r1 + 1);
94
95	In this case, the data dependency extends from the READ_ONCE()
96	on line 1 to the WRITE_ONCE() on line 2.  Data dependencies are
97	fragile and can be easily destroyed by optimizing compilers.
98	Because optimizing compilers put a great deal of effort into
99	working out what values integer variables might have, this is
100	especially true in cases where the dependency is carried through
101	an integer.
102
103	See also "Address Dependency" and "Control Dependency".
104
105From-Reads (fr):  When one CPU's store to a given variable happened
106	too late to affect the value returned by another CPU's
107	load from that same variable, there is said to be a from-reads
108	link from the load to the store.
109
110	It is also possible to have a from-reads link within a CPU, which
111	is a "from-reads internal" (fri) link.  The term "from-reads
112	external" (fre) link is used when it is necessary to exclude
113	the fri case.
114
115	See also "Coherence" and "Reads-from".
116
117Fully Ordered:  An operation such as smp_mb() that orders all of
118	its CPU's prior accesses with all of that CPU's subsequent
119	accesses, or a marked access such as atomic_add_return()
120	that orders all of its CPU's prior accesses, itself, and
121	all of its CPU's subsequent accesses.
122
123Happens-Before (hb): A relation between two accesses in which LKMM
124	guarantees the first access precedes the second.  For more
125	detail, please see the "THE HAPPENS-BEFORE RELATION: hb"
126	section of explanation.txt.
127
128Marked Access:  An access to a variable that uses an special function or
129	macro such as "r1 = READ_ONCE(x)" or "smp_store_release(&a, 1)".
130
131	See also "Unmarked Access".
132
133Pairing: "Memory-barrier pairing" reflects the fact that synchronizing
134	data between two CPUs requires that both CPUs their accesses.
135	Memory barriers thus tend to come in pairs, one executed by
136	one of the CPUs and the other by the other CPU.  Of course,
137	pairing also occurs with other types of operations, so that a
138	smp_store_release() pairs with an smp_load_acquire() that reads
139	the value stored.
140
141	See also "Cycle".
142
143Reads-From (rf):  When one CPU's load returns the value stored by some other
144	CPU, there is said to be a reads-from link from the second
145	CPU's store to the first CPU's load.  Reads-from links have the
146	nice property that time must advance from the store to the load,
147	which means that algorithms using reads-from links can use lighter
148	weight ordering and synchronization compared to algorithms using
149	coherence and from-reads links.
150
151	It is also possible to have a reads-from link within a CPU, which
152	is a "reads-from internal" (rfi) link.	The term "reads-from
153	external" (rfe) link is used when it is necessary to exclude
154	the rfi case.
155
156	See also Coherence" and "From-reads".
157
158Relaxed:  A marked access that does not imply ordering, for example, a
159	READ_ONCE(), WRITE_ONCE(), a non-value-returning read-modify-write
160	operation, or a value-returning read-modify-write operation whose
161	name ends in "_relaxed".
162
163	See also "Acquire" and "Release".
164
165Release:  With respect to a lock, releasing that lock, for example,
166	using spin_unlock().  With respect to a non-lock shared variable,
167	a special operation that includes a store and which orders that
168	store after earlier memory references that ran on that same CPU.
169	An example special release store is smp_store_release(), but
170	atomic_set_release() and atomic_cmpxchg_release() also include
171	release stores.
172
173	See also "Acquire" and "Relaxed".
174
175Unmarked Access:  An access to a variable that uses normal C-language
176	syntax, for example, "a = b[2]";
177
178	See also "Marked Access".
179