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; |
| 25 | spin_lock_init(&sem->wait_lock); |
| 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 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 39 | /* |
| 40 | * handle the lock release when processes blocked on it that can now run |
| 41 | * - if we come here from up_xxxx(), then: |
| 42 | * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed) |
| 43 | * - 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^] | 44 | * - there must be someone on the queue |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 45 | * - the spinlock must be held by the caller |
| 46 | * - woken process blocks are discarded from the list after having task zeroed |
| 47 | * - writers are only woken if downgrading is false |
| 48 | */ |
| 49 | static inline struct rw_semaphore * |
| 50 | __rwsem_do_wake(struct rw_semaphore *sem, int downgrading) |
| 51 | { |
| 52 | struct rwsem_waiter *waiter; |
| 53 | struct task_struct *tsk; |
| 54 | struct list_head *next; |
| 55 | signed long oldcount, woken, loop; |
| 56 | |
Michel Lespinasse | 345af7b | 2010-08-09 17:21:15 -0700 | [diff] [blame^] | 57 | waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); |
| 58 | if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE)) |
| 59 | goto readers_only; |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 60 | |
Michel Lespinasse | 345af7b | 2010-08-09 17:21:15 -0700 | [diff] [blame^] | 61 | if (downgrading) |
| 62 | goto out; |
| 63 | |
| 64 | /* There's a writer at the front of the queue - try to grant it the |
| 65 | * write lock. However, we only wake this writer if we can transition |
| 66 | * the active part of the count from 0 -> 1 |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 67 | */ |
Michel Lespinasse | 345af7b | 2010-08-09 17:21:15 -0700 | [diff] [blame^] | 68 | try_again_write: |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 69 | oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem) |
| 70 | - RWSEM_ACTIVE_BIAS; |
| 71 | if (oldcount & RWSEM_ACTIVE_MASK) |
Michel Lespinasse | 345af7b | 2010-08-09 17:21:15 -0700 | [diff] [blame^] | 72 | /* Someone grabbed the sem already */ |
| 73 | goto undo_write; |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 74 | |
| 75 | /* We must be careful not to touch 'waiter' after we set ->task = NULL. |
| 76 | * It is an allocated on the waiter's stack and may become invalid at |
| 77 | * any time after that point (due to a wakeup from another source). |
| 78 | */ |
| 79 | list_del(&waiter->list); |
| 80 | tsk = waiter->task; |
akpm@osdl.org | d59dd46 | 2005-05-01 08:58:47 -0700 | [diff] [blame] | 81 | smp_mb(); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 82 | waiter->task = NULL; |
| 83 | wake_up_process(tsk); |
| 84 | put_task_struct(tsk); |
| 85 | goto out; |
| 86 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 87 | readers_only: |
Michel Lespinasse | 345af7b | 2010-08-09 17:21:15 -0700 | [diff] [blame^] | 88 | if (downgrading) |
| 89 | goto wake_readers; |
| 90 | |
| 91 | /* if we came through an up_xxxx() call, we only only wake someone up |
| 92 | * if we can transition the active part of the count from 0 -> 1 */ |
| 93 | try_again_read: |
| 94 | oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem) |
| 95 | - RWSEM_ACTIVE_BIAS; |
| 96 | if (oldcount & RWSEM_ACTIVE_MASK) |
| 97 | /* Someone grabbed the sem already */ |
| 98 | goto undo_read; |
| 99 | |
| 100 | wake_readers: |
| 101 | /* Grant an infinite number of read locks to the readers at the front |
| 102 | * of the queue. Note we increment the 'active part' of the count by |
| 103 | * the number of readers before waking any processes up. |
| 104 | */ |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 105 | woken = 0; |
| 106 | do { |
| 107 | woken++; |
| 108 | |
| 109 | if (waiter->list.next == &sem->wait_list) |
| 110 | break; |
| 111 | |
| 112 | waiter = list_entry(waiter->list.next, |
| 113 | struct rwsem_waiter, list); |
| 114 | |
| 115 | } while (waiter->flags & RWSEM_WAITING_FOR_READ); |
| 116 | |
| 117 | loop = woken; |
| 118 | woken *= RWSEM_ACTIVE_BIAS - RWSEM_WAITING_BIAS; |
| 119 | if (!downgrading) |
| 120 | /* we'd already done one increment earlier */ |
| 121 | woken -= RWSEM_ACTIVE_BIAS; |
| 122 | |
| 123 | rwsem_atomic_add(woken, sem); |
| 124 | |
| 125 | next = sem->wait_list.next; |
| 126 | for (; loop > 0; loop--) { |
| 127 | waiter = list_entry(next, struct rwsem_waiter, list); |
| 128 | next = waiter->list.next; |
| 129 | tsk = waiter->task; |
akpm@osdl.org | d59dd46 | 2005-05-01 08:58:47 -0700 | [diff] [blame] | 130 | smp_mb(); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 131 | waiter->task = NULL; |
| 132 | wake_up_process(tsk); |
| 133 | put_task_struct(tsk); |
| 134 | } |
| 135 | |
| 136 | sem->wait_list.next = next; |
| 137 | next->prev = &sem->wait_list; |
| 138 | |
| 139 | out: |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 140 | return sem; |
| 141 | |
Michel Lespinasse | 91af708 | 2010-05-12 11:38:45 +0100 | [diff] [blame] | 142 | /* undo the change to the active count, but check for a transition |
| 143 | * 1->0 */ |
Michel Lespinasse | 345af7b | 2010-08-09 17:21:15 -0700 | [diff] [blame^] | 144 | undo_write: |
Michel Lespinasse | 91af708 | 2010-05-12 11:38:45 +0100 | [diff] [blame] | 145 | if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK) |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 146 | goto out; |
Michel Lespinasse | 345af7b | 2010-08-09 17:21:15 -0700 | [diff] [blame^] | 147 | goto try_again_write; |
| 148 | undo_read: |
| 149 | if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK) |
| 150 | goto out; |
| 151 | goto try_again_read; |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 152 | } |
| 153 | |
| 154 | /* |
| 155 | * wait for a lock to be granted |
| 156 | */ |
Livio Soares | c7af77b | 2007-12-18 15:21:13 +0100 | [diff] [blame] | 157 | static struct rw_semaphore __sched * |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 158 | rwsem_down_failed_common(struct rw_semaphore *sem, |
| 159 | struct rwsem_waiter *waiter, signed long adjustment) |
| 160 | { |
| 161 | struct task_struct *tsk = current; |
| 162 | signed long count; |
| 163 | |
| 164 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); |
| 165 | |
| 166 | /* set up my own style of waitqueue */ |
| 167 | spin_lock_irq(&sem->wait_lock); |
| 168 | waiter->task = tsk; |
| 169 | get_task_struct(tsk); |
| 170 | |
| 171 | list_add_tail(&waiter->list, &sem->wait_list); |
| 172 | |
| 173 | /* we're now waiting on the lock, but no longer actively read-locking */ |
| 174 | count = rwsem_atomic_update(adjustment, sem); |
| 175 | |
| 176 | /* if there are no active locks, wake the front queued process(es) up */ |
| 177 | if (!(count & RWSEM_ACTIVE_MASK)) |
| 178 | sem = __rwsem_do_wake(sem, 0); |
| 179 | |
| 180 | spin_unlock_irq(&sem->wait_lock); |
| 181 | |
| 182 | /* wait to be given the lock */ |
| 183 | for (;;) { |
| 184 | if (!waiter->task) |
| 185 | break; |
| 186 | schedule(); |
| 187 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); |
| 188 | } |
| 189 | |
| 190 | tsk->state = TASK_RUNNING; |
| 191 | |
| 192 | return sem; |
| 193 | } |
| 194 | |
| 195 | /* |
| 196 | * wait for the read lock to be granted |
| 197 | */ |
Ingo Molnar | d50efc6 | 2008-01-30 13:33:00 +0100 | [diff] [blame] | 198 | asmregparm struct rw_semaphore __sched * |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 199 | rwsem_down_read_failed(struct rw_semaphore *sem) |
| 200 | { |
| 201 | struct rwsem_waiter waiter; |
| 202 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 203 | waiter.flags = RWSEM_WAITING_FOR_READ; |
| 204 | rwsem_down_failed_common(sem, &waiter, |
| 205 | RWSEM_WAITING_BIAS - RWSEM_ACTIVE_BIAS); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 206 | return sem; |
| 207 | } |
| 208 | |
| 209 | /* |
| 210 | * wait for the write lock to be granted |
| 211 | */ |
Ingo Molnar | d50efc6 | 2008-01-30 13:33:00 +0100 | [diff] [blame] | 212 | asmregparm struct rw_semaphore __sched * |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 213 | rwsem_down_write_failed(struct rw_semaphore *sem) |
| 214 | { |
| 215 | struct rwsem_waiter waiter; |
| 216 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 217 | waiter.flags = RWSEM_WAITING_FOR_WRITE; |
| 218 | rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_BIAS); |
| 219 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 220 | return sem; |
| 221 | } |
| 222 | |
| 223 | /* |
| 224 | * handle waking up a waiter on the semaphore |
| 225 | * - up_read/up_write has decremented the active part of count if we come here |
| 226 | */ |
Ingo Molnar | d50efc6 | 2008-01-30 13:33:00 +0100 | [diff] [blame] | 227 | asmregparm struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 228 | { |
| 229 | unsigned long flags; |
| 230 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 231 | spin_lock_irqsave(&sem->wait_lock, flags); |
| 232 | |
| 233 | /* do nothing if list empty */ |
| 234 | if (!list_empty(&sem->wait_list)) |
| 235 | sem = __rwsem_do_wake(sem, 0); |
| 236 | |
| 237 | spin_unlock_irqrestore(&sem->wait_lock, flags); |
| 238 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 239 | return sem; |
| 240 | } |
| 241 | |
| 242 | /* |
| 243 | * downgrade a write lock into a read lock |
| 244 | * - caller incremented waiting part of count and discovered it still negative |
| 245 | * - just wake up any readers at the front of the queue |
| 246 | */ |
Ingo Molnar | d50efc6 | 2008-01-30 13:33:00 +0100 | [diff] [blame] | 247 | asmregparm struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem) |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 248 | { |
| 249 | unsigned long flags; |
| 250 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 251 | spin_lock_irqsave(&sem->wait_lock, flags); |
| 252 | |
| 253 | /* do nothing if list empty */ |
| 254 | if (!list_empty(&sem->wait_list)) |
| 255 | sem = __rwsem_do_wake(sem, 1); |
| 256 | |
| 257 | spin_unlock_irqrestore(&sem->wait_lock, flags); |
| 258 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 259 | return sem; |
| 260 | } |
| 261 | |
| 262 | EXPORT_SYMBOL(rwsem_down_read_failed); |
| 263 | EXPORT_SYMBOL(rwsem_down_write_failed); |
| 264 | EXPORT_SYMBOL(rwsem_wake); |
| 265 | EXPORT_SYMBOL(rwsem_downgrade_wake); |