1 /* SPDX-License-Identifier: GPL-2.0 */ 2 #ifndef _LINUX_SWAIT_H 3 #define _LINUX_SWAIT_H 4 5 #include <linux/list.h> 6 #include <linux/stddef.h> 7 #include <linux/spinlock.h> 8 #include <linux/wait.h> 9 #include <asm/current.h> 10 11 /* 12 * BROKEN wait-queues. 13 * 14 * These "simple" wait-queues are broken garbage, and should never be 15 * used. The comments below claim that they are "similar" to regular 16 * wait-queues, but the semantics are actually completely different, and 17 * every single user we have ever had has been buggy (or pointless). 18 * 19 * A "swake_up_one()" only wakes up _one_ waiter, which is not at all what 20 * "wake_up()" does, and has led to problems. In other cases, it has 21 * been fine, because there's only ever one waiter (kvm), but in that 22 * case gthe whole "simple" wait-queue is just pointless to begin with, 23 * since there is no "queue". Use "wake_up_process()" with a direct 24 * pointer instead. 25 * 26 * While these are very similar to regular wait queues (wait.h) the most 27 * important difference is that the simple waitqueue allows for deterministic 28 * behaviour -- IOW it has strictly bounded IRQ and lock hold times. 29 * 30 * Mainly, this is accomplished by two things. Firstly not allowing swake_up_all 31 * from IRQ disabled, and dropping the lock upon every wakeup, giving a higher 32 * priority task a chance to run. 33 * 34 * Secondly, we had to drop a fair number of features of the other waitqueue 35 * code; notably: 36 * 37 * - mixing INTERRUPTIBLE and UNINTERRUPTIBLE sleeps on the same waitqueue; 38 * all wakeups are TASK_NORMAL in order to avoid O(n) lookups for the right 39 * sleeper state. 40 * 41 * - the !exclusive mode; because that leads to O(n) wakeups, everything is 42 * exclusive. 43 * 44 * - custom wake callback functions; because you cannot give any guarantees 45 * about random code. This also allows swait to be used in RT, such that 46 * raw spinlock can be used for the swait queue head. 47 * 48 * As a side effect of these; the data structures are slimmer albeit more ad-hoc. 49 * For all the above, note that simple wait queues should _only_ be used under 50 * very specific realtime constraints -- it is best to stick with the regular 51 * wait queues in most cases. 52 */ 53 54 struct task_struct; 55 56 struct swait_queue_head { 57 raw_spinlock_t lock; 58 struct list_head task_list; 59 }; 60 61 struct swait_queue { 62 struct task_struct *task; 63 struct list_head task_list; 64 }; 65 66 #define __SWAITQUEUE_INITIALIZER(name) { \ 67 .task = current, \ 68 .task_list = LIST_HEAD_INIT((name).task_list), \ 69 } 70 71 #define DECLARE_SWAITQUEUE(name) \ 72 struct swait_queue name = __SWAITQUEUE_INITIALIZER(name) 73 74 #define __SWAIT_QUEUE_HEAD_INITIALIZER(name) { \ 75 .lock = __RAW_SPIN_LOCK_UNLOCKED(name.lock), \ 76 .task_list = LIST_HEAD_INIT((name).task_list), \ 77 } 78 79 #define DECLARE_SWAIT_QUEUE_HEAD(name) \ 80 struct swait_queue_head name = __SWAIT_QUEUE_HEAD_INITIALIZER(name) 81 82 extern void __init_swait_queue_head(struct swait_queue_head *q, const char *name, 83 struct lock_class_key *key); 84 85 #define init_swait_queue_head(q) \ 86 do { \ 87 static struct lock_class_key __key; \ 88 __init_swait_queue_head((q), #q, &__key); \ 89 } while (0) 90 91 #ifdef CONFIG_LOCKDEP 92 # define __SWAIT_QUEUE_HEAD_INIT_ONSTACK(name) \ 93 ({ init_swait_queue_head(&name); name; }) 94 # define DECLARE_SWAIT_QUEUE_HEAD_ONSTACK(name) \ 95 struct swait_queue_head name = __SWAIT_QUEUE_HEAD_INIT_ONSTACK(name) 96 #else 97 # define DECLARE_SWAIT_QUEUE_HEAD_ONSTACK(name) \ 98 DECLARE_SWAIT_QUEUE_HEAD(name) 99 #endif 100 101 /** 102 * swait_active -- locklessly test for waiters on the queue 103 * @wq: the waitqueue to test for waiters 104 * 105 * returns true if the wait list is not empty 106 * 107 * NOTE: this function is lockless and requires care, incorrect usage _will_ 108 * lead to sporadic and non-obvious failure. 109 * 110 * NOTE2: this function has the same above implications as regular waitqueues. 111 * 112 * Use either while holding swait_queue_head::lock or when used for wakeups 113 * with an extra smp_mb() like: 114 * 115 * CPU0 - waker CPU1 - waiter 116 * 117 * for (;;) { 118 * @cond = true; prepare_to_swait_exclusive(&wq_head, &wait, state); 119 * smp_mb(); // smp_mb() from set_current_state() 120 * if (swait_active(wq_head)) if (@cond) 121 * wake_up(wq_head); break; 122 * schedule(); 123 * } 124 * finish_swait(&wq_head, &wait); 125 * 126 * Because without the explicit smp_mb() it's possible for the 127 * swait_active() load to get hoisted over the @cond store such that we'll 128 * observe an empty wait list while the waiter might not observe @cond. 129 * This, in turn, can trigger missing wakeups. 130 * 131 * Also note that this 'optimization' trades a spin_lock() for an smp_mb(), 132 * which (when the lock is uncontended) are of roughly equal cost. 133 */ 134 static inline int swait_active(struct swait_queue_head *wq) 135 { 136 return !list_empty(&wq->task_list); 137 } 138 139 /** 140 * swq_has_sleeper - check if there are any waiting processes 141 * @wq: the waitqueue to test for waiters 142 * 143 * Returns true if @wq has waiting processes 144 * 145 * Please refer to the comment for swait_active. 146 */ 147 static inline bool swq_has_sleeper(struct swait_queue_head *wq) 148 { 149 /* 150 * We need to be sure we are in sync with the list_add() 151 * modifications to the wait queue (task_list). 152 * 153 * This memory barrier should be paired with one on the 154 * waiting side. 155 */ 156 smp_mb(); 157 return swait_active(wq); 158 } 159 160 extern void swake_up_one(struct swait_queue_head *q); 161 extern void swake_up_all(struct swait_queue_head *q); 162 extern void swake_up_locked(struct swait_queue_head *q); 163 164 extern void prepare_to_swait_exclusive(struct swait_queue_head *q, struct swait_queue *wait, int state); 165 extern long prepare_to_swait_event(struct swait_queue_head *q, struct swait_queue *wait, int state); 166 167 extern void __finish_swait(struct swait_queue_head *q, struct swait_queue *wait); 168 extern void finish_swait(struct swait_queue_head *q, struct swait_queue *wait); 169 170 /* as per ___wait_event() but for swait, therefore "exclusive == 1" */ 171 #define ___swait_event(wq, condition, state, ret, cmd) \ 172 ({ \ 173 __label__ __out; \ 174 struct swait_queue __wait; \ 175 long __ret = ret; \ 176 \ 177 INIT_LIST_HEAD(&__wait.task_list); \ 178 for (;;) { \ 179 long __int = prepare_to_swait_event(&wq, &__wait, state);\ 180 \ 181 if (condition) \ 182 break; \ 183 \ 184 if (___wait_is_interruptible(state) && __int) { \ 185 __ret = __int; \ 186 goto __out; \ 187 } \ 188 \ 189 cmd; \ 190 } \ 191 finish_swait(&wq, &__wait); \ 192 __out: __ret; \ 193 }) 194 195 #define __swait_event(wq, condition) \ 196 (void)___swait_event(wq, condition, TASK_UNINTERRUPTIBLE, 0, \ 197 schedule()) 198 199 #define swait_event_exclusive(wq, condition) \ 200 do { \ 201 if (condition) \ 202 break; \ 203 __swait_event(wq, condition); \ 204 } while (0) 205 206 #define __swait_event_timeout(wq, condition, timeout) \ 207 ___swait_event(wq, ___wait_cond_timeout(condition), \ 208 TASK_UNINTERRUPTIBLE, timeout, \ 209 __ret = schedule_timeout(__ret)) 210 211 #define swait_event_timeout_exclusive(wq, condition, timeout) \ 212 ({ \ 213 long __ret = timeout; \ 214 if (!___wait_cond_timeout(condition)) \ 215 __ret = __swait_event_timeout(wq, condition, timeout); \ 216 __ret; \ 217 }) 218 219 #define __swait_event_interruptible(wq, condition) \ 220 ___swait_event(wq, condition, TASK_INTERRUPTIBLE, 0, \ 221 schedule()) 222 223 #define swait_event_interruptible_exclusive(wq, condition) \ 224 ({ \ 225 int __ret = 0; \ 226 if (!(condition)) \ 227 __ret = __swait_event_interruptible(wq, condition); \ 228 __ret; \ 229 }) 230 231 #define __swait_event_interruptible_timeout(wq, condition, timeout) \ 232 ___swait_event(wq, ___wait_cond_timeout(condition), \ 233 TASK_INTERRUPTIBLE, timeout, \ 234 __ret = schedule_timeout(__ret)) 235 236 #define swait_event_interruptible_timeout_exclusive(wq, condition, timeout)\ 237 ({ \ 238 long __ret = timeout; \ 239 if (!___wait_cond_timeout(condition)) \ 240 __ret = __swait_event_interruptible_timeout(wq, \ 241 condition, timeout); \ 242 __ret; \ 243 }) 244 245 #define __swait_event_idle(wq, condition) \ 246 (void)___swait_event(wq, condition, TASK_IDLE, 0, schedule()) 247 248 /** 249 * swait_event_idle_exclusive - wait without system load contribution 250 * @wq: the waitqueue to wait on 251 * @condition: a C expression for the event to wait for 252 * 253 * The process is put to sleep (TASK_IDLE) until the @condition evaluates to 254 * true. The @condition is checked each time the waitqueue @wq is woken up. 255 * 256 * This function is mostly used when a kthread or workqueue waits for some 257 * condition and doesn't want to contribute to system load. Signals are 258 * ignored. 259 */ 260 #define swait_event_idle_exclusive(wq, condition) \ 261 do { \ 262 if (condition) \ 263 break; \ 264 __swait_event_idle(wq, condition); \ 265 } while (0) 266 267 #define __swait_event_idle_timeout(wq, condition, timeout) \ 268 ___swait_event(wq, ___wait_cond_timeout(condition), \ 269 TASK_IDLE, timeout, \ 270 __ret = schedule_timeout(__ret)) 271 272 /** 273 * swait_event_idle_timeout_exclusive - wait up to timeout without load contribution 274 * @wq: the waitqueue to wait on 275 * @condition: a C expression for the event to wait for 276 * @timeout: timeout at which we'll give up in jiffies 277 * 278 * The process is put to sleep (TASK_IDLE) until the @condition evaluates to 279 * true. The @condition is checked each time the waitqueue @wq is woken up. 280 * 281 * This function is mostly used when a kthread or workqueue waits for some 282 * condition and doesn't want to contribute to system load. Signals are 283 * ignored. 284 * 285 * Returns: 286 * 0 if the @condition evaluated to %false after the @timeout elapsed, 287 * 1 if the @condition evaluated to %true after the @timeout elapsed, 288 * or the remaining jiffies (at least 1) if the @condition evaluated 289 * to %true before the @timeout elapsed. 290 */ 291 #define swait_event_idle_timeout_exclusive(wq, condition, timeout) \ 292 ({ \ 293 long __ret = timeout; \ 294 if (!___wait_cond_timeout(condition)) \ 295 __ret = __swait_event_idle_timeout(wq, \ 296 condition, timeout); \ 297 __ret; \ 298 }) 299 300 #endif /* _LINUX_SWAIT_H */ 301