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  * 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 */