diff options
Diffstat (limited to 'include/linux/swait.h')
| -rw-r--r-- | include/linux/swait.h | 169 |
1 files changed, 142 insertions, 27 deletions
diff --git a/include/linux/swait.h b/include/linux/swait.h index c1f9c62a8a50..d324419482a0 100644 --- a/include/linux/swait.h +++ b/include/linux/swait.h @@ -1,35 +1,41 @@ +/* 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> /* - * Simple wait queues - * - * While these are very similar to the other/complex 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 + * Simple waitqueues are semantically very different 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. * - * In order to make this so, we had to drop a fair number of features of the - * other waitqueue code; notably: + * 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 this requires preserving the list order - * and this is hard. - * - * - custom wake functions; because you cannot give any guarantees about - * random code. + * - the !exclusive mode; because that leads to O(n) wakeups, everything is + * exclusive. As such swake_up_one will only ever awake _one_ waiter. * - * As a side effect of this; the data structures are slimmer. + * - 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. * - * One would recommend using this wait queue where possible. + * 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; @@ -79,25 +85,79 @@ extern void __init_swait_queue_head(struct swait_queue_head *q, const char *name DECLARE_SWAIT_QUEUE_HEAD(name) #endif -static inline int swait_active(struct swait_queue_head *q) +/** + * 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) { - return !list_empty(&q->task_list); + /* + * 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(struct swait_queue_head *q); +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 swake_up_locked(struct swait_queue_head *q, int wake_flags); -extern void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait); -extern void prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait, int state); +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 == 0" */ +/* 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; \ \ @@ -110,20 +170,20 @@ extern void finish_swait(struct swait_queue_head *q, struct swait_queue *wait); \ if (___wait_is_interruptible(state) && __int) { \ __ret = __int; \ - break; \ + goto __out; \ } \ \ cmd; \ } \ finish_swait(&wq, &__wait); \ - __ret; \ +__out: __ret; \ }) #define __swait_event(wq, condition) \ (void)___swait_event(wq, condition, TASK_UNINTERRUPTIBLE, 0, \ schedule()) -#define swait_event(wq, condition) \ +#define swait_event_exclusive(wq, condition) \ do { \ if (condition) \ break; \ @@ -135,7 +195,7 @@ do { \ TASK_UNINTERRUPTIBLE, timeout, \ __ret = schedule_timeout(__ret)) -#define swait_event_timeout(wq, condition, timeout) \ +#define swait_event_timeout_exclusive(wq, condition, timeout) \ ({ \ long __ret = timeout; \ if (!___wait_cond_timeout(condition)) \ @@ -147,7 +207,7 @@ do { \ ___swait_event(wq, condition, TASK_INTERRUPTIBLE, 0, \ schedule()) -#define swait_event_interruptible(wq, condition) \ +#define swait_event_interruptible_exclusive(wq, condition) \ ({ \ int __ret = 0; \ if (!(condition)) \ @@ -160,7 +220,7 @@ do { \ TASK_INTERRUPTIBLE, timeout, \ __ret = schedule_timeout(__ret)) -#define swait_event_interruptible_timeout(wq, condition, timeout) \ +#define swait_event_interruptible_timeout_exclusive(wq, condition, timeout)\ ({ \ long __ret = timeout; \ if (!___wait_cond_timeout(condition)) \ @@ -169,4 +229,59 @@ do { \ __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 */ |