include/linux/swait.h

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