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Based on kernel version 3.15.4. Page generated on 2014-07-07 08:59 EST.

1	vlocks for Bare-Metal Mutual Exclusion
2	======================================
3	
4	Voting Locks, or "vlocks" provide a simple low-level mutual exclusion
5	mechanism, with reasonable but minimal requirements on the memory
6	system.
7	
8	These are intended to be used to coordinate critical activity among CPUs
9	which are otherwise non-coherent, in situations where the hardware
10	provides no other mechanism to support this and ordinary spinlocks
11	cannot be used.
12	
13	
14	vlocks make use of the atomicity provided by the memory system for
15	writes to a single memory location.  To arbitrate, every CPU "votes for
16	itself", by storing a unique number to a common memory location.  The
17	final value seen in that memory location when all the votes have been
18	cast identifies the winner.
19	
20	In order to make sure that the election produces an unambiguous result
21	in finite time, a CPU will only enter the election in the first place if
22	no winner has been chosen and the election does not appear to have
23	started yet.
24	
25	
26	Algorithm
27	---------
28	
29	The 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	
67	The currently_voting[] array provides a way for the CPUs to determine
68	whether an election is in progress, and plays a role analogous to the
69	"entering" array in Lamport's bakery algorithm [1].
70	
71	However, once the election has started, the underlying memory system
72	atomicity is used to pick the winner.  This avoids the need for a static
73	priority rule to act as a tie-breaker, or any counters which could
74	overflow.
75	
76	As long as the last_vote variable is globally visible to all CPUs, it
77	will contain only one value that won't change once every CPU has cleared
78	its currently_voting flag.
79	
80	
81	Features 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	
118	ARM implementation
119	------------------
120	
121	The current ARM implementation [2] contains some optimisations beyond
122	the 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	
189	Colophon
190	--------
191	
192	Originally created and documented by Dave Martin for Linaro Limited, for
193	use in ARM-based big.LITTLE platforms, with review and input gratefully
194	received from Nicolas Pitre and Achin Gupta.  Thanks to Nicolas for
195	grabbing most of this text out of the relevant mail thread and writing
196	up the pseudocode.
197	
198	Copyright (C) 2012-2013  Linaro Limited
199	Distributed under the terms of Version 2 of the GNU General Public
200	License, as defined in linux/COPYING.
201	
202	
203	References
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.
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