diff options
| -rw-r--r-- | include/bakery_lock.h | 2 | ||||
| -rw-r--r-- | lib/sync/locks/bakery/bakery_lock.c | 162 |
2 files changed, 131 insertions, 33 deletions
diff --git a/include/bakery_lock.h b/include/bakery_lock.h index c7ff903..da5d9dd 100644 --- a/include/bakery_lock.h +++ b/include/bakery_lock.h @@ -37,7 +37,7 @@ #ifndef __ASSEMBLY__ typedef struct { - volatile int owner; + int owner; volatile char entering[BAKERY_LOCK_MAX_CPUS]; volatile unsigned number[BAKERY_LOCK_MAX_CPUS]; } bakery_lock; diff --git a/lib/sync/locks/bakery/bakery_lock.c b/lib/sync/locks/bakery/bakery_lock.c index 444b6a1..ec08135 100644 --- a/lib/sync/locks/bakery/bakery_lock.c +++ b/lib/sync/locks/bakery/bakery_lock.c @@ -32,64 +32,162 @@ #include <string.h> #include <bakery_lock.h> +#include <arch_helpers.h> + +/* + * Functions in this file implement Bakery Algorithm for mutual exclusion. + * + * 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); \ assert(entry < BAKERY_LOCK_MAX_CPUS); \ } while(0) +/* Convert a ticket to priority */ +#define PRIORITY(t, pos) (((t) << 8) | (pos)) + + +/* Initialize Bakery Lock to reset ownership and all ticket values */ void bakery_lock_init(bakery_lock * bakery) { assert(bakery); + + /* All ticket values need to be 0 */ memset(bakery, 0, sizeof(*bakery)); bakery->owner = NO_OWNER; } -void bakery_lock_get(unsigned long mpidr, bakery_lock * bakery) + +/* Obtain a ticket for a given CPU */ +static unsigned int bakery_get_ticket(bakery_lock *bakery, unsigned int me) { - unsigned int i, max = 0, my_full_number, his_full_number, entry; + unsigned int my_ticket, their_ticket; + unsigned int they; - entry = platform_get_core_pos(mpidr); + /* + * 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 = 0; + bakery->entering[me] = 1; + for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) { + their_ticket = bakery->number[they]; + if (their_ticket > my_ticket) + my_ticket = their_ticket; + } - assert_bakery_entry_valid(entry, bakery); + /* + * Compute ticket; then signal to other contenders waiting for us to + * finish calculating our ticket value that we're done + */ + ++my_ticket; + bakery->number[me] = my_ticket; + bakery->entering[me] = 0; + sev(); - // Catch recursive attempts to take the lock under the same entry: - assert(bakery->owner != entry); + return my_ticket; +} - // Get a ticket - bakery->entering[entry] = 1; - for (i = 0; i < BAKERY_LOCK_MAX_CPUS; ++i) { - if (bakery->number[i] > max) { - max = bakery->number[i]; - } - } - ++max; - bakery->number[entry] = max; - bakery->entering[entry] = 0; - - // Wait for our turn - my_full_number = (max << 8) + entry; - for (i = 0; i < BAKERY_LOCK_MAX_CPUS; ++i) { - while (bakery->entering[i]) ; /* Wait */ - do { - his_full_number = bakery->number[i]; - if (his_full_number) { - his_full_number = (his_full_number << 8) + i; - } + +/* + * 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(unsigned long mpidr, bakery_lock *bakery) +{ + unsigned int they, me; + unsigned int my_ticket, my_prio, their_ticket; + + me = platform_get_core_pos(mpidr); + + assert_bakery_entry_valid(me, bakery); + + /* Prevent recursive acquisition */ + assert(bakery->owner != me); + + /* 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 = PRIORITY(my_ticket, me); + for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) { + if (me == they) + continue; + + /* Wait for the contender to get their ticket */ + while (bakery->entering[they]) + wfe(); + + /* + * If the other party is a contender, they'll have non-zero + * (valid) ticket value. If they do, compare priorities + */ + their_ticket = bakery->number[they]; + if (their_ticket && (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 { + wfe(); + } while (their_ticket == bakery->number[they]); } - while (his_full_number && (his_full_number < my_full_number)); } - bakery->owner = entry; + /* Lock acquired */ + bakery->owner = me; } + +/* Release the lock and signal contenders */ void bakery_lock_release(unsigned long mpidr, bakery_lock * bakery) { - unsigned int entry = platform_get_core_pos(mpidr); + unsigned int me = platform_get_core_pos(mpidr); - assert_bakery_entry_valid(entry, bakery); - assert(bakery->owner == entry); + assert_bakery_entry_valid(me, bakery); + assert(bakery->owner == me); + /* + * Release lock by resetting ownership and ticket. Then signal other + * waiting contenders + */ bakery->owner = NO_OWNER; - bakery->number[entry] = 0; + bakery->number[me] = 0; + sev(); } |