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/* SPDX-License-Identifier: GPL-2.0 / #ifndef _LINUX_SWAIT_H #define _LINUX_SWAIT_H #include <linux/list.h> #include <linux/stddef.h> #include <linux/spinlock.h> #include <linux/wait.h> #include <asm/current.h> / * BROKEN wait-queues. * * These "simple" wait-queues are broken garbage, and should never be * used. The comments below claim that they are "similar" to regular * wait-queues, but the semantics are actually completely different, and * every single user we have ever had has been buggy (or pointless). * * A "swake_up_one()" only wakes up _one_ waiter, which is not at all what * "wake_up()" does, and has led to problems. In other cases, it has * been fine, because there's only ever one waiter (kvm), but in that * case gthe whole "simple" wait-queue is just pointless to begin with, * since there is no "queue". Use "wake_up_process()" with a direct * pointer instead. * * While these are very similar to regular wait queues (wait.h) the most * important difference is that the simple waitqueue allows for deterministic * behaviour -- IOW it has strictly bounded IRQ and lock hold times. * * Mainly, this is accomplished by two things. Firstly not allowing swake_up_all * from IRQ disabled, and dropping the lock upon every wakeup, giving a higher * priority task a chance to run. * * Secondly, we had to drop a fair number of features of the other waitqueue * code; notably: * * - mixing INTERRUPTIBLE and UNINTERRUPTIBLE sleeps on the same waitqueue; * all wakeups are TASK_NORMAL in order to avoid O(n) lookups for the right * sleeper state. * * - the !exclusive mode; because that leads to O(n) wakeups, everything is * exclusive. * * - custom wake callback functions; because you cannot give any guarantees * about random code. This also allows swait to be used in RT, such that * raw spinlock can be used for the swait queue head. * * As a side effect of these; the data structures are slimmer albeit more ad-hoc. * For all the above, note that simple wait queues should _only_ be used under * very specific realtime constraints -- it is best to stick with the regular * wait queues in most cases. / struct task_struct; struct swait_queue_head { raw_spinlock_t lock; struct list_head task_list; }; struct swait_queue { struct task_struct task; struct list_head task_list; }; #define __SWAITQUEUE_INITIALIZER(name) { \ .task = current, \ .task_list = LIST_HEAD_INIT((name).task_list), \ } #define DECLARE_SWAITQUEUE(name) \ struct swait_queue name = __SWAITQUEUE_INITIALIZER(name) #define __SWAIT_QUEUE_HEAD_INITIALIZER(name) { \ .lock = __RAW_SPIN_LOCK_UNLOCKED(name.lock), \ .task_list = LIST_HEAD_INIT((name).task_list), \ } #define DECLARE_SWAIT_QUEUE_HEAD(name) \ struct swait_queue_head name = __SWAIT_QUEUE_HEAD_INITIALIZER(name) extern void __init_swait_queue_head(struct swait_queue_head q, const char name, struct lock_class_key key); #define init_swait_queue_head(q) \ do { \ static struct lock_class_key __key; \ __init_swait_queue_head((q), #q, &__key); \ } while (0) #ifdef CONFIG_LOCKDEP # define __SWAIT_QUEUE_HEAD_INIT_ONSTACK(name) \ ({ init_swait_queue_head(&name); name; }) # define DECLARE_SWAIT_QUEUE_HEAD_ONSTACK(name) \ struct swait_queue_head name = __SWAIT_QUEUE_HEAD_INIT_ONSTACK(name) #else # define DECLARE_SWAIT_QUEUE_HEAD_ONSTACK(name) \ DECLARE_SWAIT_QUEUE_HEAD(name) #endif /* * swait_active -- locklessly test for waiters on the queue * @wq: the waitqueue to test for waiters * * returns true if the wait list is not empty * * NOTE: this function is lockless and requires care, incorrect usage _will_ * lead to sporadic and non-obvious failure. * * NOTE2: this function has the same above implications as regular waitqueues. * * Use either while holding swait_queue_head::lock or when used for wakeups * with an extra smp_mb() like: * * CPU0 - waker CPU1 - waiter * * for (;;) { * @cond = true; prepare_to_swait_exclusive(&wq_head, &wait, state); * smp_mb(); // smp_mb() from set_current_state() * if (swait_active(wq_head)) if (@cond) * wake_up(wq_head); break; * schedule(); * } * finish_swait(&wq_head, &wait); * * Because without the explicit smp_mb() it's possible for the * swait_active() load to get hoisted over the @cond store such that we'll * observe an empty wait list while the waiter might not observe @cond. * This, in turn, can trigger missing wakeups. * * Also note that this 'optimization' trades a spin_lock() for an smp_mb(), * which (when the lock is uncontended) are of roughly equal cost. / static inline int swait_active(struct swait_queue_head wq) { return !list_empty(&wq->task_list); } /** * swq_has_sleeper - check if there are any waiting processes * @wq: the waitqueue to test for waiters * * Returns true if @wq has waiting processes * * Please refer to the comment for swait_active. / static inline bool swq_has_sleeper(struct swait_queue_head wq) { /* * We need to be sure we are in sync with the list_add() * modifications to the wait queue (task_list). * * This memory barrier should be paired with one on the * waiting side. / smp_mb(); return swait_active(wq); } extern void swake_up_one(struct swait_queue_head q); extern void swake_up_all(struct swait_queue_head q); extern void swake_up_locked(struct swait_queue_head q); extern void prepare_to_swait_exclusive(struct swait_queue_head q, struct swait_queue wait, int state); extern long prepare_to_swait_event(struct swait_queue_head q, struct swait_queue wait, int state); extern void __finish_swait(struct swait_queue_head q, struct swait_queue wait); extern void finish_swait(struct swait_queue_head q, struct swait_queue wait); /* as per ___wait_event() but for swait, therefore "exclusive == 1" / #define ___swait_event(wq, condition, state, ret, cmd) \ ({ \ __label__ __out; \ struct swait_queue __wait; \ long __ret = ret; \ \ INIT_LIST_HEAD(&__wait.task_list); \ for (;;) { \ long __int = prepare_to_swait_event(&wq, &__wait, state);\ \ if (condition) \ break; \ \ if (___wait_is_interruptible(state) && __int) { \ __ret = __int; \ goto __out; \ } \ \ cmd; \ } \ finish_swait(&wq, &__wait); \ __out: __ret; \ }) #define __swait_event(wq, condition) \ (void)___swait_event(wq, condition, TASK_UNINTERRUPTIBLE, 0, \ schedule()) #define swait_event_exclusive(wq, condition) \ do { \ if (condition) \ break; \ __swait_event(wq, condition); \ } while (0) #define __swait_event_timeout(wq, condition, timeout) \ ___swait_event(wq, ___wait_cond_timeout(condition), \ TASK_UNINTERRUPTIBLE, timeout, \ __ret = schedule_timeout(__ret)) #define swait_event_timeout_exclusive(wq, condition, timeout) \ ({ \ long __ret = timeout; \ if (!___wait_cond_timeout(condition)) \ __ret = __swait_event_timeout(wq, condition, timeout); \ __ret; \ }) #define __swait_event_interruptible(wq, condition) \ ___swait_event(wq, condition, TASK_INTERRUPTIBLE, 0, \ schedule()) #define swait_event_interruptible_exclusive(wq, condition) \ ({ \ int __ret = 0; \ if (!(condition)) \ __ret = __swait_event_interruptible(wq, condition); \ __ret; \ }) #define __swait_event_interruptible_timeout(wq, condition, timeout) \ ___swait_event(wq, ___wait_cond_timeout(condition), \ TASK_INTERRUPTIBLE, timeout, \ __ret = schedule_timeout(__ret)) #define swait_event_interruptible_timeout_exclusive(wq, condition, timeout)\ ({ \ long __ret = timeout; \ if (!___wait_cond_timeout(condition)) \ __ret = __swait_event_interruptible_timeout(wq, \ condition, timeout); \ __ret; \ }) #define __swait_event_idle(wq, condition) \ (void)___swait_event(wq, condition, TASK_IDLE, 0, schedule()) /* * swait_event_idle_exclusive - wait without system load contribution * @wq: the waitqueue to wait on * @condition: a C expression for the event to wait for * * The process is put to sleep (TASK_IDLE) until the @condition evaluates to * true. The @condition is checked each time the waitqueue @wq is woken up. * * This function is mostly used when a kthread or workqueue waits for some * condition and doesn't want to contribute to system load. Signals are * ignored. / #define swait_event_idle_exclusive(wq, condition) \ do { \ if (condition) \ break; \ __swait_event_idle(wq, condition); \ } while (0) #define __swait_event_idle_timeout(wq, condition, timeout) \ ___swait_event(wq, ___wait_cond_timeout(condition), \ TASK_IDLE, timeout, \ __ret = schedule_timeout(__ret)) /* * swait_event_idle_timeout_exclusive - wait up to timeout without load contribution * @wq: the waitqueue to wait on * @condition: a C expression for the event to wait for * @timeout: timeout at which we'll give up in jiffies * * The process is put to sleep (TASK_IDLE) until the @condition evaluates to * true. The @condition is checked each time the waitqueue @wq is woken up. * * This function is mostly used when a kthread or workqueue waits for some * condition and doesn't want to contribute to system load. Signals are * ignored. * * Returns: * 0 if the @condition evaluated to %false after the @timeout elapsed, * 1 if the @condition evaluated to %true after the @timeout elapsed, * or the remaining jiffies (at least 1) if the @condition evaluated * to %true before the @timeout elapsed. / #define swait_event_idle_timeout_exclusive(wq, condition, timeout) \ ({ \ long __ret = timeout; \ if (!___wait_cond_timeout(condition)) \ __ret = __swait_event_idle_timeout(wq, \ condition, timeout); \ __ret; \ }) #endif / _LINUX_SWAIT_H */

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