lib/locks/bakery/bakery_lock_normal.c

   1 /*
   2  * Copyright (c) 2015-2018, ARM Limited and Contributors. All rights reserved.
   3  *
   4  * SPDX-License-Identifier: BSD-3-Clause
   5  */
   6
   7 #include <assert.h>
   8 #include <string.h>
   9
  10 #include <arch_helpers.h>
  11 #include <lib/bakery_lock.h>
  12 #include <lib/el3_runtime/cpu_data.h>
  13 #include <lib/utils_def.h>
  14 #include <plat/common/platform.h>
  15
  16 /*
  17  * Functions in this file implement Bakery Algorithm for mutual exclusion with the
  18  * bakery lock data structures in cacheable and Normal memory.
  19  *
  20  * ARM architecture offers a family of exclusive access instructions to
  21  * efficiently implement mutual exclusion with hardware support. However, as
  22  * well as depending on external hardware, these instructions have defined
  23  * behavior only on certain memory types (cacheable and Normal memory in
  24  * particular; see ARMv8 Architecture Reference Manual section B2.10). Use cases
  25  * in trusted firmware are such that mutual exclusion implementation cannot
  26  * expect that accesses to the lock have the specific type required by the
  27  * architecture for these primitives to function (for example, not all
  28  * contenders may have address translation enabled).
  29  *
  30  * This implementation does not use mutual exclusion primitives. It expects
  31  * memory regions where the locks reside to be cacheable and Normal.
  32  *
  33  * Note that the ARM architecture guarantees single-copy atomicity for aligned
  34  * accesses regardless of status of address translation.
  35  */
  36
  37 #ifdef PLAT_PERCPU_BAKERY_LOCK_SIZE
  38 /*
  39  * Verify that the platform defined value for the per-cpu space for bakery locks is
  40  * a multiple of the cache line size, to prevent multiple CPUs writing to the same
  41  * bakery lock cache line
  42  *
  43  * Using this value, if provided, rather than the linker generated value results in
  44  * more efficient code
  45  */
  46 CASSERT((PLAT_PERCPU_BAKERY_LOCK_SIZE & (CACHE_WRITEBACK_GRANULE - 1)) == 0, \
  47         PLAT_PERCPU_BAKERY_LOCK_SIZE_not_cacheline_multiple);
  48 #define PERCPU_BAKERY_LOCK_SIZE (PLAT_PERCPU_BAKERY_LOCK_SIZE)
  49 #else
  50 /*
  51  * Use the linker defined symbol which has evaluated the size reqiurement.
  52  * This is not as efficient as using a platform defined constant
  53  */
  54 IMPORT_SYM(uintptr_t, __PERCPU_BAKERY_LOCK_SIZE__, PERCPU_BAKERY_LOCK_SIZE);
  55 #endif
  56
  57 static inline bakery_lock_t *get_bakery_info(unsigned int cpu_ix,
  58                                              bakery_lock_t *lock)
  59 {
  60         return (bakery_info_t *)((uintptr_t)lock +
  61                                 cpu_ix * PERCPU_BAKERY_LOCK_SIZE);
  62 }
  63
  64 static inline void write_cache_op(uintptr_t addr, bool cached)
  65 {
  66         if (cached)
  67                 dccvac(addr);
  68         else
  69                 dcivac(addr);
  70
  71         dsbish();
  72 }
  73
  74 static inline void read_cache_op(uintptr_t addr, bool cached)
  75 {
  76         if (cached)
  77                 dccivac(addr);
  78 }
  79
  80 /* Helper function to check if the lock is acquired */
  81 static inline bool is_lock_acquired(const bakery_info_t *my_bakery_info,
  82                                                         int is_cached)
  83 {
  84         /*
  85          * Even though lock data is updated only by the owning cpu and
  86          * appropriate cache maintenance operations are performed,
  87          * if the previous update was done when the cpu was not participating
  88          * in coherency, then there is a chance that cache maintenance
  89          * operations were not propagated to all the caches in the system.
  90          * Hence do a `read_cache_op()` prior to read.
  91          */
  92         read_cache_op((uintptr_t)my_bakery_info, is_cached);
  93         return bakery_ticket_number(my_bakery_info->lock_data) != 0U;
  94 }
  95
  96 static unsigned int bakery_get_ticket(bakery_lock_t *lock,
  97                                                 unsigned int me, int is_cached)
  98 {
  99         unsigned int my_ticket, their_ticket;
 100         unsigned int they;
 101         bakery_info_t *my_bakery_info, *their_bakery_info;
 102
 103         /*
 104          * Obtain a reference to the bakery information for this cpu and ensure
 105          * it is not NULL.
 106          */
 107         my_bakery_info = get_bakery_info(me, lock);
 108         assert(my_bakery_info != NULL);
 109
 110         /* Prevent recursive acquisition.*/
 111         assert(!is_lock_acquired(my_bakery_info, is_cached));
 112
 113         /*
 114          * Tell other contenders that we are through the bakery doorway i.e.
 115          * going to allocate a ticket for this cpu.
 116          */
 117         my_ticket = 0U;
 118         my_bakery_info->lock_data = make_bakery_data(CHOOSING_TICKET, my_ticket);
 119
 120         write_cache_op((uintptr_t)my_bakery_info, is_cached);
 121
 122         /*
 123          * Iterate through the bakery information of each contender to allocate
 124          * the highest ticket number for this cpu.
 125          */
 126         for (they = 0U; they < BAKERY_LOCK_MAX_CPUS; they++) {
 127                 if (me == they)
 128                         continue;
 129
 130                 /*
 131                  * Get a reference to the other contender's bakery info and
 132                  * ensure that a stale copy is not read.
 133                  */
 134                 their_bakery_info = get_bakery_info(they, lock);
 135                 assert(their_bakery_info != NULL);
 136
 137                 read_cache_op((uintptr_t)their_bakery_info, is_cached);
 138
 139                 /*
 140                  * Update this cpu's ticket number if a higher ticket number is
 141                  * seen
 142                  */
 143                 their_ticket = bakery_ticket_number(their_bakery_info->lock_data);
 144                 if (their_ticket > my_ticket)
 145                         my_ticket = their_ticket;
 146         }
 147
 148         /*
 149          * Compute ticket; then signal to other contenders waiting for us to
 150          * finish calculating our ticket value that we're done
 151          */
 152         ++my_ticket;
 153         my_bakery_info->lock_data = make_bakery_data(CHOSEN_TICKET, my_ticket);
 154
 155         write_cache_op((uintptr_t)my_bakery_info, is_cached);
 156
 157         return my_ticket;
 158 }
 159
 160 void bakery_lock_get(bakery_lock_t *lock)
 161 {
 162         unsigned int they, me, is_cached;
 163         unsigned int my_ticket, my_prio, their_ticket;
 164         bakery_info_t *their_bakery_info;
 165         unsigned int their_bakery_data;
 166
 167         me = plat_my_core_pos();
 168 #ifdef AARCH32
 169         is_cached = read_sctlr() & SCTLR_C_BIT;
 170 #else
 171         is_cached = read_sctlr_el3() & SCTLR_C_BIT;
 172 #endif
 173
 174         /* Get a ticket */
 175         my_ticket = bakery_get_ticket(lock, me, is_cached);
 176
 177         /*
 178          * Now that we got our ticket, compute our priority value, then compare
 179          * with that of others, and proceed to acquire the lock
 180          */
 181         my_prio = bakery_get_priority(my_ticket, me);
 182         for (they = 0U; they < BAKERY_LOCK_MAX_CPUS; they++) {
 183                 if (me == they)
 184                         continue;
 185
 186                 /*
 187                  * Get a reference to the other contender's bakery info and
 188                  * ensure that a stale copy is not read.
 189                  */
 190                 their_bakery_info = get_bakery_info(they, lock);
 191                 assert(their_bakery_info != NULL);
 192
 193                 /* Wait for the contender to get their ticket */
 194                 do {
 195                         read_cache_op((uintptr_t)their_bakery_info, is_cached);
 196                         their_bakery_data = their_bakery_info->lock_data;
 197                 } while (bakery_is_choosing(their_bakery_data));
 198
 199                 /*
 200                  * If the other party is a contender, they'll have non-zero
 201                  * (valid) ticket value. If they do, compare priorities
 202                  */
 203                 their_ticket = bakery_ticket_number(their_bakery_data);
 204                 if (their_ticket && (bakery_get_priority(their_ticket, they) < my_prio)) {
 205                         /*
 206                          * They have higher priority (lower value). Wait for
 207                          * their ticket value to change (either release the lock
 208                          * to have it dropped to 0; or drop and probably content
 209                          * again for the same lock to have an even higher value)
 210                          */
 211                         do {
 212                                 wfe();
 213                                 read_cache_op((uintptr_t)their_bakery_info, is_cached);
 214                         } while (their_ticket
 215                                 == bakery_ticket_number(their_bakery_info->lock_data));
 216                 }
 217         }
 218
 219         /*
 220          * Lock acquired. Ensure that any reads from a shared resource in the
 221          * critical section read values after the lock is acquired.
 222          */
 223         dmbld();
 224 }
 225
 226 void bakery_lock_release(bakery_lock_t *lock)
 227 {
 228         bakery_info_t *my_bakery_info;
 229 #ifdef AARCH32
 230         unsigned int is_cached = read_sctlr() & SCTLR_C_BIT;
 231 #else
 232         unsigned int is_cached = read_sctlr_el3() & SCTLR_C_BIT;
 233 #endif
 234
 235         my_bakery_info = get_bakery_info(plat_my_core_pos(), lock);
 236
 237         assert(is_lock_acquired(my_bakery_info, is_cached));
 238
 239         /*
 240          * Ensure that other observers see any stores in the critical section
 241          * before releasing the lock. Release the lock by resetting ticket.
 242          * Then signal other waiting contenders.
 243          */
 244         dmbst();
 245         my_bakery_info->lock_data = 0U;
 246         write_cache_op((uintptr_t)my_bakery_info, is_cached);
 247         sev();
 248 }