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Date: Wed, 14 Jan 2015 11:38:34 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Paul Gortmaker <paul.gortmaker@...driver.com>
Cc: linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
Thomas Gleixner <tglx@...utronix.de>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 3/7] wait.[ch]: Introduce the simple waitqueue (swait)
implementation
So I had a look at this yesterday and came up with the below --
completely untested etc.
Now in order to compile test I meant to convert the completion code and
ran head first into complete_all; it uses spin_lock_irqsave() which
means it can be used from IRQ context. Now if you look at
__swake_up_all() you'll find a comment on how we cannot have this.
Now I can't remember how important that all was for RT but I figured I'd
post it and let other people stare at it for a bit.
---
include/linux/swait.h | 181 ++++++++++++++++++++++++++++++++++++++++++++++++++
kernel/sched/swait.c | 162 ++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 343 insertions(+)
--- /dev/null
+++ b/include/linux/swait.h
@@ -0,0 +1,181 @@
+#ifndef _LINUX_SWAIT_H
+#define _LINUX_SWAIT_H
+
+#include <linux/list.h>
+#include <linux/stddef.h>
+#include <linux/spinlock.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
+ * times.
+ *
+ * In order to make this so, we had to drop a fair number of features of the
+ * other waitqueue code; notably:
+ *
+ * - mixing INTERRUPTIBLE and UNINTERRUPTIBLE sleeps on the same waitqueue
+ *
+ * - the exclusive mode; because this requires preserving the list order
+ * and this is hard, see __swait_wake().
+ *
+ * - custom wake functions; because you cannot give any guarantees about
+ * random code.
+ *
+ * As a side effect of this; the data structures are slimmer.
+ *
+ * One would recommend using this wait queue where possible.
+ */
+
+struct task_struct;
+
+struct swait_queue_head {
+ raw_spinlock_t lock;
+#ifdef CONFIG_SWAIT_DEBUG
+ unsigned int state;
+#endif
+ 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_DEBUG_INIT() \
+ .state = 0,
+
+#define __SWAIT_QUEUE_HEAD_INITIALIZER(name) { \
+ .lock = __RAW_SPIN_LOCK_UNLOCKED(name.lock), \
+ .task_list = LIST_HEAD_INIT((name).task_list), \
+ __SWAIT_QUEUE_HEAD_DEBUG_INIT() \
+}
+
+#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
+
+static inline int swait_active(struct swait_queue_head *q)
+{
+ return !list_empty(&q->task_list);
+}
+
+extern void __swake_up(struct swait_queue_head *q, unsigned int mode);
+extern void __swake_up_all(struct swait_queue_head *q, unsigned int mode);
+extern void __swake_up_locked(struct swait_queue_head *q, unsigned int mode);
+
+#define swake_up(x) __swake_up(x, TASK_NORMAL)
+#define swake_up_all(x) __swake_up_all(x, TASK_NORMAL)
+#define swake_up_locked(x) __swake_up_locked((x), TASK_NORMAL)
+
+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 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" */
+#define ___swait_event(wq, condition, state, ret, cmd) \
+({ \
+ 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; \
+ break; \
+ } \
+ \
+ cmd; \
+ } \
+ finish_swait(&wq, &__wait); \
+ __ret; \
+})
+
+#define __swait_event(wq, condition) \
+ (void)___swait_event(wq, condition, TASK_UNINTERRUPTIBLE, 0, \
+ schedule())
+
+#define swait_event(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(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(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(wq, condition, timeout) \
+({ \
+ long __ret = timeout; \
+ if (!___wait_cond_timeout(condition)) \
+ __ret = __swait_event_interruptible_timeout(wq, \
+ condition, timeout); \
+ __ret; \
+})
+
+#endif /* _LINUX_SWAIT_H */
--- /dev/null
+++ b/kernel/sched/swait.c
@@ -0,0 +1,162 @@
+
+#include <linux/swait.h>
+
+void __init_swait_queue_head(struct swait_queue_head *q, const char *name,
+ struct lock_class_key *key)
+{
+ raw_spin_lock_init(&q->lock);
+ lockdep_set_class_and_name(&q->lock, key, name);
+ INIT_LIST_HEAD(&q->task_list);
+#ifdef CONFIG_SWAIT_DEBUG
+ q->state = 0;
+#endif
+}
+EXPORT_SYMBOL(__init_swait_queue_head);
+
+#ifdef CONFIG_SWAIT_DEBUG
+/*
+ * Ensure we do not mix and match INTERRUPTIBLE and UNINTERRUPTIBLE sleeps.
+ * This guarantees wakeups are always valid and we need not go look for
+ * wakeup targets, this ensures __swake_up() is O(1).
+ */
+static inline void __swait_wakeup_debug(struct swait_queue_head *q, unsigned int state)
+{
+ if (q->state == 0)
+ return;
+
+ WARN_ON_ONCE(!(q->state & state));
+}
+static inline void __swait_wait_debug(struct swait_queue_head *q, unsigned int state)
+{
+ if (q->state == 0)
+ q->state = state;
+
+ WARN_ON_ONCE(q->state != state);
+}
+#else
+static inline void __swait_wakeup_debug(struct swait_queue_head *q, unsigned int state)
+{
+}
+static inline void __swait_wait_debug(struct swait_queue_head *q, unsigned int state)
+{
+}
+#endif
+
+/*
+ * The thing about the wake_up_state() return value; I think we can ignore it.
+ *
+ * If for some reason it would return 0, that means the previously waiting
+ * task is already running, so it will observe condition true (or has already).
+ */
+void __swake_up_locked(struct swait_queue_head *q, unsigned int state)
+{
+ struct swait_queue *curr;
+
+ __swait_wakeup_debug(q, state);
+
+ list_for_each_entry(curr, &q->task_list, task_list) {
+ wake_up_state(curr->task, state);
+ list_del_init(&curr->task_list);
+ break;
+ }
+}
+EXPORT_SYMBOL(__swake_up_locked);
+
+void __swake_up(struct swait_queue_head *q, unsigned int state)
+{
+ unsigned long flags;
+
+ __swait_wakeup_debug(q, state);
+
+ if (!swait_active(q))
+ return;
+
+ raw_spin_lock_irqsave(&q->lock, flags);
+ __swake_up_locked(q, state);
+ raw_spin_unlock_irqrestore(&q->lock, flags);
+}
+EXPORT_SYMBOL(__swake_up);
+
+/*
+ * Does not allow usage from IRQ disabled, since we must be able to
+ * release IRQs to guarantee bounded hold time.
+ */
+void __swake_up_all(struct swait_queue_head *q, unsigned int state)
+{
+ struct swait_queue *curr, *next;
+ LIST_HEAD(tmp);
+
+ __swait_wakeup_debug(q, state);
+
+ if (!swait_active(q))
+ return;
+
+ raw_spin_lock_irq(&q->lock);
+ list_splice_init(&q->task_list, &tmp);
+ while (!list_empty(&tmp)) {
+ curr = list_first_entry(&tmp, typeof(curr), task_list);
+
+ wake_up_state(curr->task, state);
+ list_del_init(&curr->task_list);
+
+ if (list_empty(&tmp))
+ break;
+
+ raw_spin_unlock_irq(&q->lock);
+ raw_spin_lock_irq(&q->lock);
+ }
+ raw_spin_unlock_irq(&q->lock);
+}
+EXPORT_SYMBOL(__swake_up_all);
+
+void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait)
+{
+ wait->task = current;
+ if (list_empty(&wait->node))
+ list_add(&wait->task_list, &q->task_list);
+}
+
+void prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait, int state)
+{
+ unsigned long flags;
+
+ __swait_wait_debug(q, state);
+
+ raw_spin_lock_irqsave(&q->lock, flags);
+ __prepare_to_swait(q, wait);
+ set_current_state(state);
+ raw_spin_unlock_irqrestore(&q->lock, flags);
+}
+EXPORT_SYMBOL(prepare_to_swait);
+
+long prepare_to_swait_event(struct swait_queue_head *q, struct swait_queue *wait, int state)
+{
+ if (signal_pending_state(state, current))
+ return -ERESTARTSYS;
+
+ prepare_to_swait(q, wait, state);
+
+ return 0;
+}
+EXPORT_SYMBOL(prepare_to_swait_event);
+
+void __finish_swait(struct swait_queue_head *q, struct swait_queue *wait)
+{
+ __set_current_state(TASK_RUNNING);
+ if (!list_empty(&wait->task_list))
+ list_del_init(&wait->task_list);
+}
+
+void finish_swait(struct swait_queue_head *q, struct swait_queue *wait)
+{
+ unsigned long flags;
+
+ __set_current_state(TASK_RUNNING);
+
+ if (!list_empty_careful(&wait->task_list)) {
+ raw_spin_lock_irqsave(&q->lock, flags);
+ list_del_init(&wait->task_list);
+ raw_spin_unlock_irqrestore(&q->lock, flags);
+ }
+}
+EXPORT_SYMBOL(finish_swait);
--
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