diff options
Diffstat (limited to 'include/linux/swait.h')
| -rw-r--r-- | include/linux/swait.h | 86 |
1 files changed, 73 insertions, 13 deletions
diff --git a/include/linux/swait.h b/include/linux/swait.h index 4a4e180d0a35..c98aaf677466 100644 --- a/include/linux/swait.h +++ b/include/linux/swait.h @@ -1,3 +1,4 @@ +/* SPDX-License-Identifier: GPL-2.0 */ #ifndef _LINUX_SWAIT_H #define _LINUX_SWAIT_H @@ -9,13 +10,16 @@ /* * 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 - * times. + * 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. * - * 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 @@ -24,12 +28,14 @@ * - 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. - * - * 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; @@ -79,9 +85,63 @@ 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(&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); |
