212 lines
6.6 KiB
Plaintext
212 lines
6.6 KiB
Plaintext
|
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.
|
||
|
|
|
||
|
|
https://en.wikipedia.org/wiki/Lamport%27s_bakery_algorithm
|
||
|
|
|
||
|
|
[2] linux/arch/arm/common/vlock.S, www.kernel.org.
|