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 11/* 12 * Initialize an rwsem: 13 */ 14void __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)); 22 lockdep_init_map(&sem->dep_map, name, key, 0); 23#endif 24 sem->count = RWSEM_UNLOCKED_VALUE; 25 spin_lock_init(&sem->wait_lock); 26 INIT_LIST_HEAD(&sem->wait_list); 27} 28 29EXPORT_SYMBOL(__init_rwsem); 30 31struct 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 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 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) 52 * - there must be someone on the queue 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 */ 57static struct rw_semaphore * 58__rwsem_do_wake(struct rw_semaphore *sem, int wake_type) 59{ 60 struct rwsem_waiter *waiter; 61 struct task_struct *tsk; 62 struct list_head *next; 63 signed long oldcount, woken, loop, adjustment; 64 65 waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); 66 if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE)) 67 goto readers_only; 68 69 if (wake_type == RWSEM_WAKE_READ_OWNED) 70 /* Another active reader was observed, so wakeup is not 71 * likely to succeed. Save the atomic op. 72 */ 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 78 */ 79 adjustment = RWSEM_ACTIVE_WRITE_BIAS; 80 if (waiter->list.next == &sem->wait_list) 81 adjustment -= RWSEM_WAITING_BIAS; 82 83 try_again_write: 84 oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; 85 if (oldcount & RWSEM_ACTIVE_MASK) 86 /* Someone grabbed the sem already */ 87 goto undo_write; 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; 95 smp_mb(); 96 waiter->task = NULL; 97 wake_up_process(tsk); 98 put_task_struct(tsk); 99 goto out; 100 101 readers_only: 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 && 117 rwsem_atomic_update(0, sem) < RWSEM_WAITING_BIAS) 118 /* Someone grabbed the sem for write already */ 119 goto out; 120 121 /* 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 */ 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); 136 137 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; 141 142 rwsem_atomic_add(adjustment, sem); 143 144 next = sem->wait_list.next; 145 for (loop = woken; loop > 0; loop--) { 146 waiter = list_entry(next, struct rwsem_waiter, list); 147 next = waiter->list.next; 148 tsk = waiter->task; 149 smp_mb(); 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: 159 return sem; 160 161 /* undo the change to the active count, but check for a transition 162 * 1->0 */ 163 undo_write: 164 if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK) 165 goto out; 166 goto try_again_write; 167} 168 169/* 170 * wait for a lock to be granted 171 */ 172static struct rw_semaphore __sched * 173rwsem_down_failed_common(struct rw_semaphore *sem, 174 unsigned int flags, signed long adjustment) 175{ 176 struct rwsem_waiter waiter; 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 */ 183 spin_lock_irq(&sem->wait_lock); 184 waiter.task = tsk; 185 waiter.flags = flags; 186 get_task_struct(tsk); 187 188 if (list_empty(&sem->wait_list)) 189 adjustment += RWSEM_WAITING_BIAS; 190 list_add_tail(&waiter.list, &sem->wait_list); 191 192 /* we're now waiting on the lock, but no longer actively locking */ 193 count = rwsem_atomic_update(adjustment, sem); 194 195 /* 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) 202 sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE); 203 else if (count > RWSEM_WAITING_BIAS && 204 adjustment == -RWSEM_ACTIVE_WRITE_BIAS) 205 sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED); 206 207 spin_unlock_irq(&sem->wait_lock); 208 209 /* wait to be given the lock */ 210 for (;;) { 211 if (!waiter.task) 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 */ 225asmregparm struct rw_semaphore __sched * 226rwsem_down_read_failed(struct rw_semaphore *sem) 227{ 228 return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_READ, 229 -RWSEM_ACTIVE_READ_BIAS); 230} 231 232/* 233 * wait for the write lock to be granted 234 */ 235asmregparm struct rw_semaphore __sched * 236rwsem_down_write_failed(struct rw_semaphore *sem) 237{ 238 return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_WRITE, 239 -RWSEM_ACTIVE_WRITE_BIAS); 240} 241 242/* 243 * handle waking up a waiter on the semaphore 244 * - up_read/up_write has decremented the active part of count if we come here 245 */ 246asmregparm struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) 247{ 248 unsigned long flags; 249 250 spin_lock_irqsave(&sem->wait_lock, flags); 251 252 /* do nothing if list empty */ 253 if (!list_empty(&sem->wait_list)) 254 sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY); 255 256 spin_unlock_irqrestore(&sem->wait_lock, flags); 257 258 return sem; 259} 260 261/* 262 * downgrade a write lock into a read lock 263 * - caller incremented waiting part of count and discovered it still negative 264 * - just wake up any readers at the front of the queue 265 */ 266asmregparm struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem) 267{ 268 unsigned long flags; 269 270 spin_lock_irqsave(&sem->wait_lock, flags); 271 272 /* do nothing if list empty */ 273 if (!list_empty(&sem->wait_list)) 274 sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED); 275 276 spin_unlock_irqrestore(&sem->wait_lock, flags); 277 278 return sem; 279} 280 281EXPORT_SYMBOL(rwsem_down_read_failed); 282EXPORT_SYMBOL(rwsem_down_write_failed); 283EXPORT_SYMBOL(rwsem_wake); 284EXPORT_SYMBOL(rwsem_downgrade_wake); 285