--- /dev/null
+vlocks for Bare-Metal Mutual Exclusion
+======================================
+
+Voting Locks, or "vlocks" provide a simple low-level mutual exclusion
+mechanism, with reasonable but minimal requirements on the memory
+system.
+
+These are intended to be used to coordinate critical activity among CPUs
+which are otherwise non-coherent, in situations where the hardware
+provides no other mechanism to support this and ordinary spinlocks
+cannot be used.
+
+
+vlocks make use of the atomicity provided by the memory system for
+writes to a single memory location. To arbitrate, every CPU "votes for
+itself", by storing a unique number to a common memory location. The
+final value seen in that memory location when all the votes have been
+cast identifies the winner.
+
+In order to make sure that the election produces an unambiguous result
+in finite time, a CPU will only enter the election in the first place if
+no winner has been chosen and the election does not appear to have
+started yet.
+
+
+Algorithm
+---------
+
+The easiest way to explain the vlocks algorithm is with some pseudo-code:
+
+
+ int currently_voting[NR_CPUS] = { 0, };
+ int last_vote = -1; /* no votes yet */
+
+ bool vlock_trylock(int this_cpu)
+ {
+ /* signal our desire to vote */
+ currently_voting[this_cpu] = 1;
+ if (last_vote != -1) {
+ /* someone already volunteered himself */
+ currently_voting[this_cpu] = 0;
+ return false; /* not ourself */
+ }
+
+ /* let's suggest ourself */
+ last_vote = this_cpu;
+ currently_voting[this_cpu] = 0;
+
+ /* then wait until everyone else is done voting */
+ for_each_cpu(i) {
+ while (currently_voting[i] != 0)
+ /* wait */;
+ }
+
+ /* result */
+ if (last_vote == this_cpu)
+ return true; /* we won */
+ return false;
+ }
+
+ bool vlock_unlock(void)
+ {
+ last_vote = -1;
+ }
+
+
+The currently_voting[] array provides a way for the CPUs to determine
+whether an election is in progress, and plays a role analogous to the
+"entering" array in Lamport's bakery algorithm [1].
+
+However, once the election has started, the underlying memory system
+atomicity is used to pick the winner. This avoids the need for a static
+priority rule to act as a tie-breaker, or any counters which could
+overflow.
+
+As long as the last_vote variable is globally visible to all CPUs, it
+will contain only one value that won't change once every CPU has cleared
+its currently_voting flag.
+
+
+Features and limitations
+------------------------
+
+ * vlocks are not intended to be fair. In the contended case, it is the
+ _last_ CPU which attempts to get the lock which will be most likely
+ to win.
+
+ vlocks are therefore best suited to situations where it is necessary
+ to pick a unique winner, but it does not matter which CPU actually
+ wins.
+
+ * Like other similar mechanisms, vlocks will not scale well to a large
+ number of CPUs.
+
+ vlocks can be cascaded in a voting hierarchy to permit better scaling
+ if necessary, as in the following hypothetical example for 4096 CPUs:
+
+ /* first level: local election */
+ my_town = towns[(this_cpu >> 4) & 0xf];
+ I_won = vlock_trylock(my_town, this_cpu & 0xf);
+ if (I_won) {
+ /* we won the town election, let's go for the state */
+ my_state = states[(this_cpu >> 8) & 0xf];
+ I_won = vlock_lock(my_state, this_cpu & 0xf));
+ if (I_won) {
+ /* and so on */
+ I_won = vlock_lock(the_whole_country, this_cpu & 0xf];
+ if (I_won) {
+ /* ... */
+ }
+ vlock_unlock(the_whole_country);
+ }
+ vlock_unlock(my_state);
+ }
+ vlock_unlock(my_town);
+
+
+ARM implementation
+------------------
+
+The current ARM implementation [2] contains some optimisations beyond
+the basic algorithm:
+
+ * By packing the members of the currently_voting array close together,
+ we can read the whole array in one transaction (providing the number
+ of CPUs potentially contending the lock is small enough). This
+ reduces the number of round-trips required to external memory.
+
+ In the ARM implementation, this means that we can use a single load
+ and comparison:
+
+ LDR Rt, [Rn]
+ CMP Rt, #0
+
+ ...in place of code equivalent to:
+
+ LDRB Rt, [Rn]
+ CMP Rt, #0
+ LDRBEQ Rt, [Rn, #1]
+ CMPEQ Rt, #0
+ LDRBEQ Rt, [Rn, #2]
+ CMPEQ Rt, #0
+ LDRBEQ Rt, [Rn, #3]
+ CMPEQ Rt, #0
+
+ This cuts down on the fast-path latency, as well as potentially
+ reducing bus contention in contended cases.
+
+ The optimisation relies on the fact that the ARM memory system
+ guarantees coherency between overlapping memory accesses of
+ different sizes, similarly to many other architectures. Note that
+ we do not care which element of currently_voting appears in which
+ bits of Rt, so there is no need to worry about endianness in this
+ optimisation.
+
+ If there are too many CPUs to read the currently_voting array in
+ one transaction then multiple transations are still required. The
+ implementation uses a simple loop of word-sized loads for this
+ case. The number of transactions is still fewer than would be
+ required if bytes were loaded individually.
+
+
+ In principle, we could aggregate further by using LDRD or LDM, but
+ to keep the code simple this was not attempted in the initial
+ implementation.
+
+
+ * vlocks are currently only used to coordinate between CPUs which are
+ unable to enable their caches yet. This means that the
+ implementation removes many of the barriers which would be required
+ when executing the algorithm in cached memory.
+
+ packing of the currently_voting array does not work with cached
+ memory unless all CPUs contending the lock are cache-coherent, due
+ to cache writebacks from one CPU clobbering values written by other
+ CPUs. (Though if all the CPUs are cache-coherent, you should be
+ probably be using proper spinlocks instead anyway).
+
+
+ * The "no votes yet" value used for the last_vote variable is 0 (not
+ -1 as in the pseudocode). This allows statically-allocated vlocks
+ to be implicitly initialised to an unlocked state simply by putting
+ them in .bss.
+
+ An offset is added to each CPU's ID for the purpose of setting this
+ variable, so that no CPU uses the value 0 for its ID.
+
+
+Colophon
+--------
+
+Originally created and documented by Dave Martin for Linaro Limited, for
+use in ARM-based big.LITTLE platforms, with review and input gratefully
+received from Nicolas Pitre and Achin Gupta. Thanks to Nicolas for
+grabbing most of this text out of the relevant mail thread and writing
+up the pseudocode.
+
+Copyright (C) 2012-2013 Linaro Limited
+Distributed under the terms of Version 2 of the GNU General Public
+License, as defined in linux/COPYING.
+
+
+References
+----------
+
+[1] Lamport, L. "A New Solution of Dijkstra's Concurrent Programming
+ Problem", Communications of the ACM 17, 8 (August 1974), 453-455.
+
+ http://en.wikipedia.org/wiki/Lamport%27s_bakery_algorithm
+
+[2] linux/arch/arm/common/vlock.S, www.kernel.org.
Code Review / kvmfornfv.git / blobdiff