![Linus Torvalds](/assets/img/avatar_default.png)
We already earlier discouraged people from using this interface in
commit 88796e7e5c
("sched/swait: Document it clearly that the swait
facilities are special and shouldn't be used"), but I just got a pull
request with a new broken user.
So make the comment *really* clear.
The swait interfaces are bad, and should not be used unless you have
some *very* strong reasons that include tons of hard performance numbers
on just why you want to use them, and you show that you actually
understand that they aren't at all like the normal wait/wakeup
interfaces.
So far, every single user has been suspect. The main user is KVM, which
is completely pointless (there is only ever one waiter, which avoids the
interface subtleties, but also means that having a queue instead of a
pointer is counter-productive and certainly not an "optimization").
So make the comments much stronger.
Not that anybody likely reads them anyway, but there's always some
slight hope that it will cause somebody to think twice.
I'd like to remove this interface entirely, but there is the theoretical
possibility that it's actually the right thing to use in some situation,
most likely some deep RT use.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
300 lines
10 KiB
C
300 lines
10 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _LINUX_SWAIT_H
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#define _LINUX_SWAIT_H
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#include <linux/list.h>
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#include <linux/stddef.h>
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#include <linux/spinlock.h>
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#include <asm/current.h>
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/*
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* BROKEN wait-queues.
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*
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* These "simple" wait-queues are broken garbage, and should never be
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* used. The comments below claim that they are "similar" to regular
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* wait-queues, but the semantics are actually completely different, and
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* every single user we have ever had has been buggy (or pointless).
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*
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* A "swake_up()" only wakes up _one_ waiter, which is not at all what
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* "wake_up()" does, and has led to problems. In other cases, it has
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* been fine, because there's only ever one waiter (kvm), but in that
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* case gthe whole "simple" wait-queue is just pointless to begin with,
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* since there is no "queue". Use "wake_up_process()" with a direct
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* pointer instead.
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*
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* While these are very similar to regular wait queues (wait.h) the most
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* important difference is that the simple waitqueue allows for deterministic
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* behaviour -- IOW it has strictly bounded IRQ and lock hold times.
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*
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* Mainly, this is accomplished by two things. Firstly not allowing swake_up_all
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* from IRQ disabled, and dropping the lock upon every wakeup, giving a higher
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* priority task a chance to run.
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*
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* Secondly, we had to drop a fair number of features of the other waitqueue
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* code; notably:
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*
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* - mixing INTERRUPTIBLE and UNINTERRUPTIBLE sleeps on the same waitqueue;
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* all wakeups are TASK_NORMAL in order to avoid O(n) lookups for the right
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* sleeper state.
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*
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* - the exclusive mode; because this requires preserving the list order
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* and this is hard.
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*
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* - custom wake callback functions; because you cannot give any guarantees
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* about random code. This also allows swait to be used in RT, such that
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* raw spinlock can be used for the swait queue head.
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*
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* As a side effect of these; the data structures are slimmer albeit more ad-hoc.
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* For all the above, note that simple wait queues should _only_ be used under
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* very specific realtime constraints -- it is best to stick with the regular
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* wait queues in most cases.
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*/
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struct task_struct;
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struct swait_queue_head {
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raw_spinlock_t lock;
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struct list_head task_list;
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};
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struct swait_queue {
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struct task_struct *task;
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struct list_head task_list;
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};
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#define __SWAITQUEUE_INITIALIZER(name) { \
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.task = current, \
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.task_list = LIST_HEAD_INIT((name).task_list), \
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}
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#define DECLARE_SWAITQUEUE(name) \
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struct swait_queue name = __SWAITQUEUE_INITIALIZER(name)
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#define __SWAIT_QUEUE_HEAD_INITIALIZER(name) { \
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.lock = __RAW_SPIN_LOCK_UNLOCKED(name.lock), \
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.task_list = LIST_HEAD_INIT((name).task_list), \
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}
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#define DECLARE_SWAIT_QUEUE_HEAD(name) \
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struct swait_queue_head name = __SWAIT_QUEUE_HEAD_INITIALIZER(name)
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extern void __init_swait_queue_head(struct swait_queue_head *q, const char *name,
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struct lock_class_key *key);
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#define init_swait_queue_head(q) \
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do { \
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static struct lock_class_key __key; \
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__init_swait_queue_head((q), #q, &__key); \
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} while (0)
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#ifdef CONFIG_LOCKDEP
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# define __SWAIT_QUEUE_HEAD_INIT_ONSTACK(name) \
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({ init_swait_queue_head(&name); name; })
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# define DECLARE_SWAIT_QUEUE_HEAD_ONSTACK(name) \
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struct swait_queue_head name = __SWAIT_QUEUE_HEAD_INIT_ONSTACK(name)
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#else
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# define DECLARE_SWAIT_QUEUE_HEAD_ONSTACK(name) \
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DECLARE_SWAIT_QUEUE_HEAD(name)
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#endif
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/**
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* swait_active -- locklessly test for waiters on the queue
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* @wq: the waitqueue to test for waiters
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*
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* returns true if the wait list is not empty
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*
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* NOTE: this function is lockless and requires care, incorrect usage _will_
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* lead to sporadic and non-obvious failure.
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*
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* NOTE2: this function has the same above implications as regular waitqueues.
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*
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* Use either while holding swait_queue_head::lock or when used for wakeups
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* with an extra smp_mb() like:
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*
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* CPU0 - waker CPU1 - waiter
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*
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* for (;;) {
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* @cond = true; prepare_to_swait(&wq_head, &wait, state);
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* smp_mb(); // smp_mb() from set_current_state()
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* if (swait_active(wq_head)) if (@cond)
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* wake_up(wq_head); break;
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* schedule();
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* }
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* finish_swait(&wq_head, &wait);
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*
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* Because without the explicit smp_mb() it's possible for the
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* swait_active() load to get hoisted over the @cond store such that we'll
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* observe an empty wait list while the waiter might not observe @cond.
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* This, in turn, can trigger missing wakeups.
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*
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* Also note that this 'optimization' trades a spin_lock() for an smp_mb(),
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* which (when the lock is uncontended) are of roughly equal cost.
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*/
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static inline int swait_active(struct swait_queue_head *wq)
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{
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return !list_empty(&wq->task_list);
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}
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/**
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* swq_has_sleeper - check if there are any waiting processes
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* @wq: the waitqueue to test for waiters
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*
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* Returns true if @wq has waiting processes
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*
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* Please refer to the comment for swait_active.
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*/
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static inline bool swq_has_sleeper(struct swait_queue_head *wq)
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{
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/*
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* We need to be sure we are in sync with the list_add()
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* modifications to the wait queue (task_list).
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*
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* This memory barrier should be paired with one on the
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* waiting side.
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*/
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smp_mb();
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return swait_active(wq);
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}
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extern void swake_up(struct swait_queue_head *q);
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extern void swake_up_all(struct swait_queue_head *q);
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extern void swake_up_locked(struct swait_queue_head *q);
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extern void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait);
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extern void prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait, int state);
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extern long prepare_to_swait_event(struct swait_queue_head *q, struct swait_queue *wait, int state);
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extern void __finish_swait(struct swait_queue_head *q, struct swait_queue *wait);
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extern void finish_swait(struct swait_queue_head *q, struct swait_queue *wait);
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/* as per ___wait_event() but for swait, therefore "exclusive == 0" */
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#define ___swait_event(wq, condition, state, ret, cmd) \
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({ \
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struct swait_queue __wait; \
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long __ret = ret; \
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\
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INIT_LIST_HEAD(&__wait.task_list); \
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for (;;) { \
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long __int = prepare_to_swait_event(&wq, &__wait, state);\
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\
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if (condition) \
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break; \
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\
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if (___wait_is_interruptible(state) && __int) { \
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__ret = __int; \
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break; \
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} \
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\
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cmd; \
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} \
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finish_swait(&wq, &__wait); \
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__ret; \
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})
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#define __swait_event(wq, condition) \
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(void)___swait_event(wq, condition, TASK_UNINTERRUPTIBLE, 0, \
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schedule())
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#define swait_event(wq, condition) \
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do { \
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if (condition) \
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break; \
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__swait_event(wq, condition); \
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} while (0)
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#define __swait_event_timeout(wq, condition, timeout) \
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___swait_event(wq, ___wait_cond_timeout(condition), \
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TASK_UNINTERRUPTIBLE, timeout, \
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__ret = schedule_timeout(__ret))
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#define swait_event_timeout(wq, condition, timeout) \
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({ \
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long __ret = timeout; \
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if (!___wait_cond_timeout(condition)) \
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__ret = __swait_event_timeout(wq, condition, timeout); \
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__ret; \
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})
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#define __swait_event_interruptible(wq, condition) \
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___swait_event(wq, condition, TASK_INTERRUPTIBLE, 0, \
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schedule())
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#define swait_event_interruptible(wq, condition) \
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({ \
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int __ret = 0; \
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if (!(condition)) \
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__ret = __swait_event_interruptible(wq, condition); \
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__ret; \
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})
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#define __swait_event_interruptible_timeout(wq, condition, timeout) \
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___swait_event(wq, ___wait_cond_timeout(condition), \
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TASK_INTERRUPTIBLE, timeout, \
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__ret = schedule_timeout(__ret))
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#define swait_event_interruptible_timeout(wq, condition, timeout) \
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({ \
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long __ret = timeout; \
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if (!___wait_cond_timeout(condition)) \
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__ret = __swait_event_interruptible_timeout(wq, \
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condition, timeout); \
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__ret; \
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})
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#define __swait_event_idle(wq, condition) \
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(void)___swait_event(wq, condition, TASK_IDLE, 0, schedule())
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/**
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* swait_event_idle - wait without system load contribution
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* @wq: the waitqueue to wait on
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* @condition: a C expression for the event to wait for
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*
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* The process is put to sleep (TASK_IDLE) until the @condition evaluates to
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* true. The @condition is checked each time the waitqueue @wq is woken up.
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*
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* This function is mostly used when a kthread or workqueue waits for some
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* condition and doesn't want to contribute to system load. Signals are
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* ignored.
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*/
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#define swait_event_idle(wq, condition) \
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do { \
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if (condition) \
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break; \
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__swait_event_idle(wq, condition); \
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} while (0)
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#define __swait_event_idle_timeout(wq, condition, timeout) \
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___swait_event(wq, ___wait_cond_timeout(condition), \
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TASK_IDLE, timeout, \
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__ret = schedule_timeout(__ret))
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/**
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* swait_event_idle_timeout - wait up to timeout without load contribution
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* @wq: the waitqueue to wait on
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* @condition: a C expression for the event to wait for
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* @timeout: timeout at which we'll give up in jiffies
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*
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* The process is put to sleep (TASK_IDLE) until the @condition evaluates to
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* true. The @condition is checked each time the waitqueue @wq is woken up.
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*
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* This function is mostly used when a kthread or workqueue waits for some
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* condition and doesn't want to contribute to system load. Signals are
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* ignored.
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*
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* Returns:
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* 0 if the @condition evaluated to %false after the @timeout elapsed,
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* 1 if the @condition evaluated to %true after the @timeout elapsed,
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* or the remaining jiffies (at least 1) if the @condition evaluated
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* to %true before the @timeout elapsed.
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*/
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#define swait_event_idle_timeout(wq, condition, timeout) \
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({ \
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long __ret = timeout; \
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if (!___wait_cond_timeout(condition)) \
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__ret = __swait_event_idle_timeout(wq, \
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condition, timeout); \
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__ret; \
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})
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#endif /* _LINUX_SWAIT_H */
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