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* Copyright (c) 2013-2018, ARM Limited and Contributors. All rights reserved.
* SPDX-License-Identifier: BSD-3-Clause
#include <assert.h>
#include <string.h>
#include <arch_helpers.h>
#include <lib/bakery_lock.h>
#include <lib/el3_runtime/cpu_data.h>
#include <plat/common/platform.h>
* Functions in this file implement Bakery Algorithm for mutual exclusion with the
* bakery lock data structures in coherent memory.
* ARM architecture offers a family of exclusive access instructions to
* efficiently implement mutual exclusion with hardware support. However, as
* well as depending on external hardware, the these instructions have defined
* behavior only on certain memory types (cacheable and Normal memory in
* particular; see ARMv8 Architecture Reference Manual section B2.10). Use cases
* in trusted firmware are such that mutual exclusion implementation cannot
* expect that accesses to the lock have the specific type required by the
* architecture for these primitives to function (for example, not all
* contenders may have address translation enabled).
* This implementation does not use mutual exclusion primitives. It expects
* memory regions where the locks reside to be fully ordered and coherent
* (either by disabling address translation, or by assigning proper attributes
* when translation is enabled).
* Note that the ARM architecture guarantees single-copy atomicity for aligned
* accesses regardless of status of address translation.
#define assert_bakery_entry_valid(_entry, _bakery) do { \
assert((_bakery) != NULL); \
assert((_entry) < BAKERY_LOCK_MAX_CPUS); \
} while (false)
/* Obtain a ticket for a given CPU */
static unsigned int bakery_get_ticket(bakery_lock_t *bakery, unsigned int me)
unsigned int my_ticket, their_ticket;
unsigned int they;
/* Prevent recursive acquisition */
assert(bakery_ticket_number(bakery->lock_data[me]) == 0U);
* Flag that we're busy getting our ticket. All CPUs are iterated in the
* order of their ordinal position to decide the maximum ticket value
* observed so far. Our priority is set to be greater than the maximum
* observed priority
* Note that it's possible that more than one contender gets the same
* ticket value. That's OK as the lock is acquired based on the priority
* value, not the ticket value alone.
my_ticket = 0U;
bakery->lock_data[me] = make_bakery_data(CHOOSING_TICKET, my_ticket);
for (they = 0U; they < BAKERY_LOCK_MAX_CPUS; they++) {
their_ticket = bakery_ticket_number(bakery->lock_data[they]);
if (their_ticket > my_ticket)
my_ticket = their_ticket;
* Compute ticket; then signal to other contenders waiting for us to
* finish calculating our ticket value that we're done
bakery->lock_data[me] = make_bakery_data(CHOSEN_TICKET, my_ticket);
return my_ticket;
* Acquire bakery lock
* Contending CPUs need first obtain a non-zero ticket and then calculate
* priority value. A contending CPU iterate over all other CPUs in the platform,
* which may be contending for the same lock, in the order of their ordinal
* position (CPU0, CPU1 and so on). A non-contending CPU will have its ticket
* (and priority) value as 0. The contending CPU compares its priority with that
* of others'. The CPU with the highest priority (lowest numerical value)
* acquires the lock
void bakery_lock_get(bakery_lock_t *bakery)
unsigned int they, me;
unsigned int my_ticket, my_prio, their_ticket;
unsigned int their_bakery_data;
me = plat_my_core_pos();
assert_bakery_entry_valid(me, bakery);
/* Get a ticket */
my_ticket = bakery_get_ticket(bakery, me);
* Now that we got our ticket, compute our priority value, then compare
* with that of others, and proceed to acquire the lock
my_prio = bakery_get_priority(my_ticket, me);
for (they = 0U; they < BAKERY_LOCK_MAX_CPUS; they++) {
if (me == they)
/* Wait for the contender to get their ticket */
do {
their_bakery_data = bakery->lock_data[they];
} while (bakery_is_choosing(their_bakery_data));
* If the other party is a contender, they'll have non-zero
* (valid) ticket value. If they do, compare priorities
their_ticket = bakery_ticket_number(their_bakery_data);
if ((their_ticket != 0U) &&
(bakery_get_priority(their_ticket, they) < my_prio)) {
* They have higher priority (lower value). Wait for
* their ticket value to change (either release the lock
* to have it dropped to 0; or drop and probably content
* again for the same lock to have an even higher value)
do {
} while (their_ticket ==
* Lock acquired. Ensure that any reads from a shared resource in the
* critical section read values after the lock is acquired.
/* Release the lock and signal contenders */
void bakery_lock_release(bakery_lock_t *bakery)
unsigned int me = plat_my_core_pos();
assert_bakery_entry_valid(me, bakery);
assert(bakery_ticket_number(bakery->lock_data[me]) != 0U);
* Ensure that other observers see any stores in the critical section
* before releasing the lock. Release the lock by resetting ticket.
* Then signal other waiting contenders.
bakery->lock_data[me] = 0U;