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