1============
2LITMUS TESTS
3============
4
5CoRR+poonceonce+Once.litmus
6 Test of read-read coherence, that is, whether or not two
7 successive reads from the same variable are ordered.
8
9CoRW+poonceonce+Once.litmus
10 Test of read-write coherence, that is, whether or not a read
11 from a given variable followed by a write to that same variable
12 are ordered.
13
14CoWR+poonceonce+Once.litmus
15 Test of write-read coherence, that is, whether or not a write
16 to a given variable followed by a read from that same variable
17 are ordered.
18
19CoWW+poonceonce.litmus
20 Test of write-write coherence, that is, whether or not two
21 successive writes to the same variable are ordered.
22
23IRIW+fencembonceonces+OnceOnce.litmus
24 Test of independent reads from independent writes with smp_mb()
25 between each pairs of reads. In other words, is smp_mb()
26 sufficient to cause two different reading processes to agree on
27 the order of a pair of writes, where each write is to a different
28 variable by a different process? This litmus test is forbidden
29 by LKMM's propagation rule.
30
31IRIW+poonceonces+OnceOnce.litmus
32 Test of independent reads from independent writes with nothing
33 between each pairs of reads. In other words, is anything at all
34 needed to cause two different reading processes to agree on the
35 order of a pair of writes, where each write is to a different
36 variable by a different process?
37
38ISA2+pooncelock+pooncelock+pombonce.litmus
39 Tests whether the ordering provided by a lock-protected S
40 litmus test is visible to an external process whose accesses are
41 separated by smp_mb(). This addition of an external process to
42 S is otherwise known as ISA2.
43
44ISA2+poonceonces.litmus
45 As below, but with store-release replaced with WRITE_ONCE()
46 and load-acquire replaced with READ_ONCE().
47
48ISA2+pooncerelease+poacquirerelease+poacquireonce.litmus
49 Can a release-acquire chain order a prior store against
50 a later load?
51
52LB+fencembonceonce+ctrlonceonce.litmus
53 Does a control dependency and an smp_mb() suffice for the
54 load-buffering litmus test, where each process reads from one
55 of two variables then writes to the other?
56
57LB+poacquireonce+pooncerelease.litmus
58 Does a release-acquire pair suffice for the load-buffering
59 litmus test, where each process reads from one of two variables then
60 writes to the other?
61
62LB+poonceonces.litmus
63 As above, but with store-release replaced with WRITE_ONCE()
64 and load-acquire replaced with READ_ONCE().
65
66MP+onceassign+derefonce.litmus
67 As below, but with rcu_assign_pointer() and an rcu_dereference().
68
69MP+polockmbonce+poacquiresilsil.litmus
70 Protect the access with a lock and an smp_mb__after_spinlock()
71 in one process, and use an acquire load followed by a pair of
72 spin_is_locked() calls in the other process.
73
74MP+polockonce+poacquiresilsil.litmus
75 Protect the access with a lock in one process, and use an
76 acquire load followed by a pair of spin_is_locked() calls
77 in the other process.
78
79MP+polocks.litmus
80 As below, but with the second access of the writer process
81 and the first access of reader process protected by a lock.
82
83MP+poonceonces.litmus
84 As below, but without the smp_rmb() and smp_wmb().
85
86MP+pooncerelease+poacquireonce.litmus
87 As below, but with a release-acquire chain.
88
89MP+porevlocks.litmus
90 As below, but with the first access of the writer process
91 and the second access of reader process protected by a lock.
92
93MP+fencewmbonceonce+fencermbonceonce.litmus
94 Does a smp_wmb() (between the stores) and an smp_rmb() (between
95 the loads) suffice for the message-passing litmus test, where one
96 process writes data and then a flag, and the other process reads
97 the flag and then the data. (This is similar to the ISA2 tests,
98 but with two processes instead of three.)
99
100R+fencembonceonces.litmus
101 This is the fully ordered (via smp_mb()) version of one of
102 the classic counterintuitive litmus tests that illustrates the
103 effects of store propagation delays.
104
105R+poonceonces.litmus
106 As above, but without the smp_mb() invocations.
107
108SB+fencembonceonces.litmus
109 This is the fully ordered (again, via smp_mb() version of store
110 buffering, which forms the core of Dekker's mutual-exclusion
111 algorithm.
112
113SB+poonceonces.litmus
114 As above, but without the smp_mb() invocations.
115
116SB+rfionceonce-poonceonces.litmus
117 This litmus test demonstrates that LKMM is not fully multicopy
118 atomic. (Neither is it other multicopy atomic.) This litmus test
119 also demonstrates the "locations" debugging aid, which designates
120 additional registers and locations to be printed out in the dump
121 of final states in the herd7 output. Without the "locations"
122 statement, only those registers and locations mentioned in the
123 "exists" clause will be printed.
124
125S+poonceonces.litmus
126 As below, but without the smp_wmb() and acquire load.
127
128S+fencewmbonceonce+poacquireonce.litmus
129 Can a smp_wmb(), instead of a release, and an acquire order
130 a prior store against a subsequent store?
131
132WRC+poonceonces+Once.litmus
133WRC+pooncerelease+fencermbonceonce+Once.litmus
134 These two are members of an extension of the MP litmus-test
135 class in which the first write is moved to a separate process.
136 The second is forbidden because smp_store_release() is
137 A-cumulative in LKMM.
138
139Z6.0+pooncelock+pooncelock+pombonce.litmus
140 Is the ordering provided by a spin_unlock() and a subsequent
141 spin_lock() sufficient to make ordering apparent to accesses
142 by a process not holding the lock?
143
144Z6.0+pooncelock+poonceLock+pombonce.litmus
145 As above, but with smp_mb__after_spinlock() immediately
146 following the spin_lock().
147
148Z6.0+pooncerelease+poacquirerelease+fencembonceonce.litmus
149 Is the ordering provided by a release-acquire chain sufficient
150 to make ordering apparent to accesses by a process that does
151 not participate in that release-acquire chain?
152
153A great many more litmus tests are available here:
154
155 https://github.com/paulmckrcu/litmus
156
157==================
158LITMUS TEST NAMING
159==================
160
161Litmus tests are usually named based on their contents, which means that
162looking at the name tells you what the litmus test does. The naming
163scheme covers litmus tests having a single cycle that passes through
164each process exactly once, so litmus tests not fitting this description
165are named on an ad-hoc basis.
166
167The structure of a litmus-test name is the litmus-test class, a plus
168sign ("+"), and one string for each process, separated by plus signs.
169The end of the name is ".litmus".
170
171The litmus-test classes may be found in the infamous test6.pdf:
172https://www.cl.cam.ac.uk/~pes20/ppc-supplemental/test6.pdf
173Each class defines the pattern of accesses and of the variables accessed.
174For example, if the one process writes to a pair of variables, and
175the other process reads from these same variables, the corresponding
176litmus-test class is "MP" (message passing), which may be found on the
177left-hand end of the second row of tests on page one of test6.pdf.
178
179The strings used to identify the actions carried out by each process are
180complex due to a desire to have short(er) names. Thus, there is a tool to
181generate these strings from a given litmus test's actions. For example,
182consider the processes from SB+rfionceonce-poonceonces.litmus:
183
184 P0(int *x, int *y)
185 {
186 int r1;
187 int r2;
188
189 WRITE_ONCE(*x, 1);
190 r1 = READ_ONCE(*x);
191 r2 = READ_ONCE(*y);
192 }
193
194 P1(int *x, int *y)
195 {
196 int r3;
197 int r4;
198
199 WRITE_ONCE(*y, 1);
200 r3 = READ_ONCE(*y);
201 r4 = READ_ONCE(*x);
202 }
203
204The next step is to construct a space-separated list of descriptors,
205interleaving descriptions of the relation between a pair of consecutive
206accesses with descriptions of the second access in the pair.
207
208P0()'s WRITE_ONCE() is read by its first READ_ONCE(), which is a
209reads-from link (rf) and internal to the P0() process. This is
210"rfi", which is an abbreviation for "reads-from internal". Because
211some of the tools string these abbreviations together with space
212characters separating processes, the first character is capitalized,
213resulting in "Rfi".
214
215P0()'s second access is a READ_ONCE(), as opposed to (for example)
216smp_load_acquire(), so next is "Once". Thus far, we have "Rfi Once".
217
218P0()'s third access is also a READ_ONCE(), but to y rather than x.
219This is related to P0()'s second access by program order ("po"),
220to a different variable ("d"), and both accesses are reads ("RR").
221The resulting descriptor is "PodRR". Because P0()'s third access is
222READ_ONCE(), we add another "Once" descriptor.
223
224A from-read ("fre") relation links P0()'s third to P1()'s first
225access, and the resulting descriptor is "Fre". P1()'s first access is
226WRITE_ONCE(), which as before gives the descriptor "Once". The string
227thus far is thus "Rfi Once PodRR Once Fre Once".
228
229The remainder of P1() is similar to P0(), which means we add
230"Rfi Once PodRR Once". Another fre links P1()'s last access to
231P0()'s first access, which is WRITE_ONCE(), so we add "Fre Once".
232The full string is thus:
233
234 Rfi Once PodRR Once Fre Once Rfi Once PodRR Once Fre Once
235
236This string can be given to the "norm7" and "classify7" tools to
237produce the name:
238
239 $ norm7 -bell linux-kernel.bell \
240 Rfi Once PodRR Once Fre Once Rfi Once PodRR Once Fre Once | \
241 sed -e 's/:.*//g'
242 SB+rfionceonce-poonceonces
243
244Adding the ".litmus" suffix: SB+rfionceonce-poonceonces.litmus
245
246The descriptors that describe connections between consecutive accesses
247within the cycle through a given litmus test can be provided by the herd7
248tool (Rfi, Po, Fre, and so on) or by the linux-kernel.bell file (Once,
249Release, Acquire, and so on).
250
251To see the full list of descriptors, execute the following command:
252
253 $ diyone7 -bell linux-kernel.bell -show edges
254