Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 1 | /* rwsem.c: R/W semaphores: contention handling functions |
| 2 | * |
| 3 | * Written by David Howells (dhowells@redhat.com). |
| 4 | * Derived from arch/i386/kernel/semaphore.c |
| 5 | */ |
| 6 | #include <linux/rwsem.h> |
| 7 | #include <linux/sched.h> |
| 8 | #include <linux/init.h> |
| 9 | #include <linux/module.h> |
| 10 | |
Ingo Molnar | 4ea2176 | 2006-07-03 00:24:53 -0700 | [diff] [blame] | 11 | /* |
| 12 | * Initialize an rwsem: |
| 13 | */ |
| 14 | void __init_rwsem(struct rw_semaphore *sem, const char *name, |
| 15 | struct lock_class_key *key) |
| 16 | { |
| 17 | #ifdef CONFIG_DEBUG_LOCK_ALLOC |
| 18 | /* |
| 19 | * Make sure we are not reinitializing a held semaphore: |
| 20 | */ |
| 21 | debug_check_no_locks_freed((void *)sem, sizeof(*sem)); |
Peter Zijlstra | 4dfbb9d | 2006-10-11 01:45:14 -0400 | [diff] [blame] | 22 | lockdep_init_map(&sem->dep_map, name, key, 0); |
Ingo Molnar | 4ea2176 | 2006-07-03 00:24:53 -0700 | [diff] [blame] | 23 | #endif |
| 24 | sem->count = RWSEM_UNLOCKED_VALUE; |
Thomas Gleixner | ddb6c9b | 2010-02-24 09:54:54 +0100 | [diff] [blame^] | 25 | raw_spin_lock_init(&sem->wait_lock); |
Ingo Molnar | 4ea2176 | 2006-07-03 00:24:53 -0700 | [diff] [blame] | 26 | INIT_LIST_HEAD(&sem->wait_list); |
| 27 | } |
| 28 | |
| 29 | EXPORT_SYMBOL(__init_rwsem); |
| 30 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 31 | struct rwsem_waiter { |
| 32 | struct list_head list; |
| 33 | struct task_struct *task; |
| 34 | unsigned int flags; |
| 35 | #define RWSEM_WAITING_FOR_READ 0x00000001 |
| 36 | #define RWSEM_WAITING_FOR_WRITE 0x00000002 |
| 37 | }; |
| 38 | |
Michel Lespinasse | 70bdc6e | 2010-08-09 17:21:17 -0700 | [diff] [blame] | 39 | /* Wake types for __rwsem_do_wake(). Note that RWSEM_WAKE_NO_ACTIVE and |
| 40 | * RWSEM_WAKE_READ_OWNED imply that the spinlock must have been kept held |
| 41 | * since the rwsem value was observed. |
| 42 | */ |
| 43 | #define RWSEM_WAKE_ANY 0 /* Wake whatever's at head of wait list */ |
| 44 | #define RWSEM_WAKE_NO_ACTIVE 1 /* rwsem was observed with no active thread */ |
| 45 | #define RWSEM_WAKE_READ_OWNED 2 /* rwsem was observed to be read owned */ |
| 46 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 47 | /* |
| 48 | * handle the lock release when processes blocked on it that can now run |
| 49 | * - if we come here from up_xxxx(), then: |
| 50 | * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed) |
| 51 | * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so) |
Michel Lespinasse | 345af7b | 2010-08-09 17:21:15 -0700 | [diff] [blame] | 52 | * - there must be someone on the queue |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 53 | * - the spinlock must be held by the caller |
| 54 | * - woken process blocks are discarded from the list after having task zeroed |
| 55 | * - writers are only woken if downgrading is false |
| 56 | */ |
Michel Lespinasse | 70bdc6e | 2010-08-09 17:21:17 -0700 | [diff] [blame] | 57 | static struct rw_semaphore * |
| 58 | __rwsem_do_wake(struct rw_semaphore *sem, int wake_type) |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 59 | { |
| 60 | struct rwsem_waiter *waiter; |
| 61 | struct task_struct *tsk; |
| 62 | struct list_head *next; |
Michel Lespinasse | fd41b33 | 2010-08-09 17:21:18 -0700 | [diff] [blame] | 63 | signed long oldcount, woken, loop, adjustment; |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 64 | |
Michel Lespinasse | 345af7b | 2010-08-09 17:21:15 -0700 | [diff] [blame] | 65 | waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); |
| 66 | if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE)) |
| 67 | goto readers_only; |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 68 | |
Michel Lespinasse | 70bdc6e | 2010-08-09 17:21:17 -0700 | [diff] [blame] | 69 | if (wake_type == RWSEM_WAKE_READ_OWNED) |
Michel Lespinasse | 424acaa | 2010-08-09 17:21:19 -0700 | [diff] [blame] | 70 | /* Another active reader was observed, so wakeup is not |
| 71 | * likely to succeed. Save the atomic op. |
| 72 | */ |
Michel Lespinasse | 345af7b | 2010-08-09 17:21:15 -0700 | [diff] [blame] | 73 | goto out; |
| 74 | |
| 75 | /* There's a writer at the front of the queue - try to grant it the |
| 76 | * write lock. However, we only wake this writer if we can transition |
| 77 | * the active part of the count from 0 -> 1 |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 78 | */ |
Michel Lespinasse | fd41b33 | 2010-08-09 17:21:18 -0700 | [diff] [blame] | 79 | adjustment = RWSEM_ACTIVE_WRITE_BIAS; |
| 80 | if (waiter->list.next == &sem->wait_list) |
| 81 | adjustment -= RWSEM_WAITING_BIAS; |
| 82 | |
Michel Lespinasse | 345af7b | 2010-08-09 17:21:15 -0700 | [diff] [blame] | 83 | try_again_write: |
Michel Lespinasse | fd41b33 | 2010-08-09 17:21:18 -0700 | [diff] [blame] | 84 | oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 85 | if (oldcount & RWSEM_ACTIVE_MASK) |
Michel Lespinasse | 345af7b | 2010-08-09 17:21:15 -0700 | [diff] [blame] | 86 | /* Someone grabbed the sem already */ |
| 87 | goto undo_write; |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 88 | |
| 89 | /* We must be careful not to touch 'waiter' after we set ->task = NULL. |
| 90 | * It is an allocated on the waiter's stack and may become invalid at |
| 91 | * any time after that point (due to a wakeup from another source). |
| 92 | */ |
| 93 | list_del(&waiter->list); |
| 94 | tsk = waiter->task; |
akpm@osdl.org | d59dd46 | 2005-05-01 08:58:47 -0700 | [diff] [blame] | 95 | smp_mb(); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 96 | waiter->task = NULL; |
| 97 | wake_up_process(tsk); |
| 98 | put_task_struct(tsk); |
| 99 | goto out; |
| 100 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 101 | readers_only: |
Michel Lespinasse | 70bdc6e | 2010-08-09 17:21:17 -0700 | [diff] [blame] | 102 | /* If we come here from up_xxxx(), another thread might have reached |
| 103 | * rwsem_down_failed_common() before we acquired the spinlock and |
| 104 | * woken up a waiter, making it now active. We prefer to check for |
| 105 | * this first in order to not spend too much time with the spinlock |
| 106 | * held if we're not going to be able to wake up readers in the end. |
| 107 | * |
| 108 | * Note that we do not need to update the rwsem count: any writer |
| 109 | * trying to acquire rwsem will run rwsem_down_write_failed() due |
| 110 | * to the waiting threads and block trying to acquire the spinlock. |
| 111 | * |
| 112 | * We use a dummy atomic update in order to acquire the cache line |
| 113 | * exclusively since we expect to succeed and run the final rwsem |
| 114 | * count adjustment pretty soon. |
| 115 | */ |
| 116 | if (wake_type == RWSEM_WAKE_ANY && |
Michel Lespinasse | 424acaa | 2010-08-09 17:21:19 -0700 | [diff] [blame] | 117 | rwsem_atomic_update(0, sem) < RWSEM_WAITING_BIAS) |
| 118 | /* Someone grabbed the sem for write already */ |
Michel Lespinasse | 70bdc6e | 2010-08-09 17:21:17 -0700 | [diff] [blame] | 119 | goto out; |
Michel Lespinasse | 345af7b | 2010-08-09 17:21:15 -0700 | [diff] [blame] | 120 | |
Michel Lespinasse | 345af7b | 2010-08-09 17:21:15 -0700 | [diff] [blame] | 121 | /* Grant an infinite number of read locks to the readers at the front |
| 122 | * of the queue. Note we increment the 'active part' of the count by |
| 123 | * the number of readers before waking any processes up. |
| 124 | */ |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 125 | woken = 0; |
| 126 | do { |
| 127 | woken++; |
| 128 | |
| 129 | if (waiter->list.next == &sem->wait_list) |
| 130 | break; |
| 131 | |
| 132 | waiter = list_entry(waiter->list.next, |
| 133 | struct rwsem_waiter, list); |
| 134 | |
| 135 | } while (waiter->flags & RWSEM_WAITING_FOR_READ); |
| 136 | |
Michel Lespinasse | fd41b33 | 2010-08-09 17:21:18 -0700 | [diff] [blame] | 137 | adjustment = woken * RWSEM_ACTIVE_READ_BIAS; |
| 138 | if (waiter->flags & RWSEM_WAITING_FOR_READ) |
| 139 | /* hit end of list above */ |
| 140 | adjustment -= RWSEM_WAITING_BIAS; |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 141 | |
Michel Lespinasse | fd41b33 | 2010-08-09 17:21:18 -0700 | [diff] [blame] | 142 | rwsem_atomic_add(adjustment, sem); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 143 | |
| 144 | next = sem->wait_list.next; |
Michel Lespinasse | fd41b33 | 2010-08-09 17:21:18 -0700 | [diff] [blame] | 145 | for (loop = woken; loop > 0; loop--) { |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 146 | waiter = list_entry(next, struct rwsem_waiter, list); |
| 147 | next = waiter->list.next; |
| 148 | tsk = waiter->task; |
akpm@osdl.org | d59dd46 | 2005-05-01 08:58:47 -0700 | [diff] [blame] | 149 | smp_mb(); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 150 | waiter->task = NULL; |
| 151 | wake_up_process(tsk); |
| 152 | put_task_struct(tsk); |
| 153 | } |
| 154 | |
| 155 | sem->wait_list.next = next; |
| 156 | next->prev = &sem->wait_list; |
| 157 | |
| 158 | out: |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 159 | return sem; |
| 160 | |
Michel Lespinasse | 91af708 | 2010-05-12 11:38:45 +0100 | [diff] [blame] | 161 | /* undo the change to the active count, but check for a transition |
| 162 | * 1->0 */ |
Michel Lespinasse | 345af7b | 2010-08-09 17:21:15 -0700 | [diff] [blame] | 163 | undo_write: |
Michel Lespinasse | fd41b33 | 2010-08-09 17:21:18 -0700 | [diff] [blame] | 164 | if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK) |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 165 | goto out; |
Michel Lespinasse | 345af7b | 2010-08-09 17:21:15 -0700 | [diff] [blame] | 166 | goto try_again_write; |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 167 | } |
| 168 | |
| 169 | /* |
| 170 | * wait for a lock to be granted |
| 171 | */ |
Livio Soares | c7af77b | 2007-12-18 15:21:13 +0100 | [diff] [blame] | 172 | static struct rw_semaphore __sched * |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 173 | rwsem_down_failed_common(struct rw_semaphore *sem, |
Michel Lespinasse | a8618a0 | 2010-08-09 17:21:20 -0700 | [diff] [blame] | 174 | unsigned int flags, signed long adjustment) |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 175 | { |
Michel Lespinasse | a8618a0 | 2010-08-09 17:21:20 -0700 | [diff] [blame] | 176 | struct rwsem_waiter waiter; |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 177 | struct task_struct *tsk = current; |
| 178 | signed long count; |
| 179 | |
| 180 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); |
| 181 | |
| 182 | /* set up my own style of waitqueue */ |
Thomas Gleixner | ddb6c9b | 2010-02-24 09:54:54 +0100 | [diff] [blame^] | 183 | raw_spin_lock_irq(&sem->wait_lock); |
Michel Lespinasse | a8618a0 | 2010-08-09 17:21:20 -0700 | [diff] [blame] | 184 | waiter.task = tsk; |
| 185 | waiter.flags = flags; |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 186 | get_task_struct(tsk); |
| 187 | |
Michel Lespinasse | fd41b33 | 2010-08-09 17:21:18 -0700 | [diff] [blame] | 188 | if (list_empty(&sem->wait_list)) |
| 189 | adjustment += RWSEM_WAITING_BIAS; |
Michel Lespinasse | a8618a0 | 2010-08-09 17:21:20 -0700 | [diff] [blame] | 190 | list_add_tail(&waiter.list, &sem->wait_list); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 191 | |
Michel Lespinasse | 70bdc6e | 2010-08-09 17:21:17 -0700 | [diff] [blame] | 192 | /* we're now waiting on the lock, but no longer actively locking */ |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 193 | count = rwsem_atomic_update(adjustment, sem); |
| 194 | |
Michel Lespinasse | 424acaa | 2010-08-09 17:21:19 -0700 | [diff] [blame] | 195 | /* If there are no active locks, wake the front queued process(es) up. |
| 196 | * |
| 197 | * Alternatively, if we're called from a failed down_write(), there |
| 198 | * were already threads queued before us and there are no active |
| 199 | * writers, the lock must be read owned; so we try to wake any read |
| 200 | * locks that were queued ahead of us. */ |
| 201 | if (count == RWSEM_WAITING_BIAS) |
Michel Lespinasse | 70bdc6e | 2010-08-09 17:21:17 -0700 | [diff] [blame] | 202 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE); |
Michel Lespinasse | 424acaa | 2010-08-09 17:21:19 -0700 | [diff] [blame] | 203 | else if (count > RWSEM_WAITING_BIAS && |
| 204 | adjustment == -RWSEM_ACTIVE_WRITE_BIAS) |
| 205 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 206 | |
Thomas Gleixner | ddb6c9b | 2010-02-24 09:54:54 +0100 | [diff] [blame^] | 207 | raw_spin_unlock_irq(&sem->wait_lock); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 208 | |
| 209 | /* wait to be given the lock */ |
| 210 | for (;;) { |
Michel Lespinasse | a8618a0 | 2010-08-09 17:21:20 -0700 | [diff] [blame] | 211 | if (!waiter.task) |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 212 | break; |
| 213 | schedule(); |
| 214 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); |
| 215 | } |
| 216 | |
| 217 | tsk->state = TASK_RUNNING; |
| 218 | |
| 219 | return sem; |
| 220 | } |
| 221 | |
| 222 | /* |
| 223 | * wait for the read lock to be granted |
| 224 | */ |
Thomas Gleixner | d123375 | 2011-01-26 21:32:01 +0100 | [diff] [blame] | 225 | struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 226 | { |
Michel Lespinasse | a8618a0 | 2010-08-09 17:21:20 -0700 | [diff] [blame] | 227 | return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_READ, |
| 228 | -RWSEM_ACTIVE_READ_BIAS); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 229 | } |
| 230 | |
| 231 | /* |
| 232 | * wait for the write lock to be granted |
| 233 | */ |
Thomas Gleixner | d123375 | 2011-01-26 21:32:01 +0100 | [diff] [blame] | 234 | struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem) |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 235 | { |
Michel Lespinasse | a8618a0 | 2010-08-09 17:21:20 -0700 | [diff] [blame] | 236 | return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_WRITE, |
| 237 | -RWSEM_ACTIVE_WRITE_BIAS); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 238 | } |
| 239 | |
| 240 | /* |
| 241 | * handle waking up a waiter on the semaphore |
| 242 | * - up_read/up_write has decremented the active part of count if we come here |
| 243 | */ |
Thomas Gleixner | d123375 | 2011-01-26 21:32:01 +0100 | [diff] [blame] | 244 | struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 245 | { |
| 246 | unsigned long flags; |
| 247 | |
Thomas Gleixner | ddb6c9b | 2010-02-24 09:54:54 +0100 | [diff] [blame^] | 248 | raw_spin_lock_irqsave(&sem->wait_lock, flags); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 249 | |
| 250 | /* do nothing if list empty */ |
| 251 | if (!list_empty(&sem->wait_list)) |
Michel Lespinasse | 70bdc6e | 2010-08-09 17:21:17 -0700 | [diff] [blame] | 252 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 253 | |
Thomas Gleixner | ddb6c9b | 2010-02-24 09:54:54 +0100 | [diff] [blame^] | 254 | raw_spin_unlock_irqrestore(&sem->wait_lock, flags); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 255 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 256 | return sem; |
| 257 | } |
| 258 | |
| 259 | /* |
| 260 | * downgrade a write lock into a read lock |
| 261 | * - caller incremented waiting part of count and discovered it still negative |
| 262 | * - just wake up any readers at the front of the queue |
| 263 | */ |
Thomas Gleixner | d123375 | 2011-01-26 21:32:01 +0100 | [diff] [blame] | 264 | struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem) |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 265 | { |
| 266 | unsigned long flags; |
| 267 | |
Thomas Gleixner | ddb6c9b | 2010-02-24 09:54:54 +0100 | [diff] [blame^] | 268 | raw_spin_lock_irqsave(&sem->wait_lock, flags); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 269 | |
| 270 | /* do nothing if list empty */ |
| 271 | if (!list_empty(&sem->wait_list)) |
Michel Lespinasse | 70bdc6e | 2010-08-09 17:21:17 -0700 | [diff] [blame] | 272 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 273 | |
Thomas Gleixner | ddb6c9b | 2010-02-24 09:54:54 +0100 | [diff] [blame^] | 274 | raw_spin_unlock_irqrestore(&sem->wait_lock, flags); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 275 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 276 | return sem; |
| 277 | } |
| 278 | |
| 279 | EXPORT_SYMBOL(rwsem_down_read_failed); |
| 280 | EXPORT_SYMBOL(rwsem_down_write_failed); |
| 281 | EXPORT_SYMBOL(rwsem_wake); |
| 282 | EXPORT_SYMBOL(rwsem_downgrade_wake); |