1 ======================================
2 vlocks for Bare-Metal Mutual Exclusion
3 ======================================
4
5 Voting Locks, or "vlocks" provide a simple low-level mutual exclusion
6 mechanism, with reasonable but minimal requirements on the memory
7 system.
8
9 These are intended to be used to coordinate critical activity among CPUs
10 which are otherwise non-coherent, in situations where the hardware
11 provides no other mechanism to support this and ordinary spinlocks
12 cannot be used.
13
14
15 vlocks make use of the atomicity provided by the memory system for
16 writes to a single memory location. To arbitrate, every CPU "votes for
17 itself", by storing a unique number to a common memory location. The
18 final value seen in that memory location when all the votes have been
19 cast identifies the winner.
20
21 In order to make sure that the election produces an unambiguous result
22 in finite time, a CPU will only enter the election in the first place if
23 no winner has been chosen and the election does not appear to have
24 started yet.
25
26
27 Algorithm
28 ---------
29
30 The easiest way to explain the vlocks algorithm is with some pseudo-code::
31
32
33 int currently_voting[NR_CPUS] = { 0, };
34 int last_vote = -1; /* no votes yet */
35
36 bool vlock_trylock(int this_cpu)
37 {
38 /* signal our desire to vote */
39 currently_voting[this_cpu] = 1;
40 if (last_vote != -1) {
41 /* someone already volunteered himself */
42 currently_voting[this_cpu] = 0;
43 return false; /* not ourself */
44 }
45
46 /* let's suggest ourself */
47 last_vote = this_cpu;
48 currently_voting[this_cpu] = 0;
49
50 /* then wait until everyone else is done voting */
51 for_each_cpu(i) {
52 while (currently_voting[i] != 0)
53 /* wait */;
54 }
55
56 /* result */
57 if (last_vote == this_cpu)
58 return true; /* we won */
59 return false;
60 }
61
62 bool vlock_unlock(void)
63 {
64 last_vote = -1;
65 }
66
67
68 The currently_voting[] array provides a way for the CPUs to determine
69 whether an election is in progress, and plays a role analogous to the
70 "entering" array in Lamport's bakery algorithm [1].
71
72 However, once the election has started, the underlying memory system
73 atomicity is used to pick the winner. This avoids the need for a static
74 priority rule to act as a tie-breaker, or any counters which could
75 overflow.
76
77 As long as the last_vote variable is globally visible to all CPUs, it
78 will contain only one value that won't change once every CPU has cleared
79 its currently_voting flag.
80
81
82 Features and limitations
83 ------------------------
84
85 * vlocks are not intended to be fair. In the contended case, it is the
86 _last_ CPU which attempts to get the lock which will be most likely
87 to win.
88
89 vlocks are therefore best suited to situations where it is necessary
90 to pick a unique winner, but it does not matter which CPU actually
91 wins.
92
93 * Like other similar mechanisms, vlocks will not scale well to a large
94 number of CPUs.
95
96 vlocks can be cascaded in a voting hierarchy to permit better scaling
97 if necessary, as in the following hypothetical example for 4096 CPUs::
98
99 /* first level: local election */
100 my_town = towns[(this_cpu >> 4) & 0xf];
101 I_won = vlock_trylock(my_town, this_cpu & 0xf);
102 if (I_won) {
103 /* we won the town election, let's go for the state */
104 my_state = states[(this_cpu >> 8) & 0xf];
105 I_won = vlock_lock(my_state, this_cpu & 0xf));
106 if (I_won) {
107 /* and so on */
108 I_won = vlock_lock(the_whole_country, this_cpu & 0xf];
109 if (I_won) {
110 /* ... */
111 }
112 vlock_unlock(the_whole_country);
113 }
114 vlock_unlock(my_state);
115 }
116 vlock_unlock(my_town);
117
118
119 ARM implementation
120 ------------------
121
122 The current ARM implementation [2] contains some optimisations beyond
123 the basic algorithm:
124
125 * By packing the members of the currently_voting array close together,
126 we can read the whole array in one transaction (providing the number
127 of CPUs potentially contending the lock is small enough). This
128 reduces the number of round-trips required to external memory.
129
130 In the ARM implementation, this means that we can use a single load
131 and comparison::
132
133 LDR Rt, [Rn]
134 CMP Rt, #0
135
136 ...in place of code equivalent to::
137
138 LDRB Rt, [Rn]
139 CMP Rt, #0
140 LDRBEQ Rt, [Rn, #1]
141 CMPEQ Rt, #0
142 LDRBEQ Rt, [Rn, #2]
143 CMPEQ Rt, #0
144 LDRBEQ Rt, [Rn, #3]
145 CMPEQ Rt, #0
146
147 This cuts down on the fast-path latency, as well as potentially
148 reducing bus contention in contended cases.
149
150 The optimisation relies on the fact that the ARM memory system
151 guarantees coherency between overlapping memory accesses of
152 different sizes, similarly to many other architectures. Note that
153 we do not care which element of currently_voting appears in which
154 bits of Rt, so there is no need to worry about endianness in this
155 optimisation.
156
157 If there are too many CPUs to read the currently_voting array in
158 one transaction then multiple transactions are still required. The
159 implementation uses a simple loop of word-sized loads for this
160 case. The number of transactions is still fewer than would be
161 required if bytes were loaded individually.
162
163
164 In principle, we could aggregate further by using LDRD or LDM, but
165 to keep the code simple this was not attempted in the initial
166 implementation.
167
168
169 * vlocks are currently only used to coordinate between CPUs which are
170 unable to enable their caches yet. This means that the
171 implementation removes many of the barriers which would be required
172 when executing the algorithm in cached memory.
173
174 packing of the currently_voting array does not work with cached
175 memory unless all CPUs contending the lock are cache-coherent, due
176 to cache writebacks from one CPU clobbering values written by other
177 CPUs. (Though if all the CPUs are cache-coherent, you should be
178 probably be using proper spinlocks instead anyway).
179
180
181 * The "no votes yet" value used for the last_vote variable is 0 (not
182 -1 as in the pseudocode). This allows statically-allocated vlocks
183 to be implicitly initialised to an unlocked state simply by putting
184 them in .bss.
185
186 An offset is added to each CPU's ID for the purpose of setting this
187 variable, so that no CPU uses the value 0 for its ID.
188
189
190 Colophon
191 --------
192
193 Originally created and documented by Dave Martin for Linaro Limited, for
194 use in ARM-based big.LITTLE platforms, with review and input gratefully
195 received from Nicolas Pitre and Achin Gupta. Thanks to Nicolas for
196 grabbing most of this text out of the relevant mail thread and writing
197 up the pseudocode.
198
199 Copyright (C) 2012-2013 Linaro Limited
200 Distributed under the terms of Version 2 of the GNU General Public
201 License, as defined in linux/COPYING.
202
203
204 References
205 ----------
206
207 [1] Lamport, L. "A New Solution of Dijkstra's Concurrent Programming
208 Problem", Communications of the ACM 17, 8 (August 1974), 453-455.
209
210 https://en.wikipedia.org/wiki/Lamport%27s_bakery_algorithm
211
212 [2] linux/arch/arm/common/vlock.S, www.kernel.org.
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