[RFC][PATCH] irq_work

[Peter Zijlstra <peterz@infradead.org>]

Something like this, but filled out with some arch code that does the
self-ipi and calls irq_work_run() should do.

No need to molest the softirq code, no need for limited vectors of any
kind.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 include/linux/irq_callback.h |   13 ++++++++
 kernel/irq_callback.c        |   66 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 79 insertions(+)

Index: linux-2.6/include/linux/irq_callback.h
===================================================================
--- /dev/null
+++ linux-2.6/include/linux/irq_callback.h
@@ -0,0 +1,13 @@
+#ifndef _LINUX_IRQ_CALLBACK_H
+#define _LINUX_IRQ_CALLBACK_H
+
+struct irq_work {
+	struct irq_work *next;
+	void (*func)(struct irq_work *);
+};
+
+int irq_work_queue(struct irq_work *entry, void (*func)(struct irq_work *));
+void irq_work_run(void);
+void irq_work_sync(struct irq_work *entry);
+
+#endif /* _LINUX_IRQ_CALLBACK_H */
Index: linux-2.6/kernel/irq_callback.c
===================================================================
--- /dev/null
+++ linux-2.6/kernel/irq_callback.c
@@ -0,0 +1,66 @@
+
+#include <linux/irq_callback.h>
+
+#define CALLBACK_TAIL ((struct irq_work *)-1UL)
+
+static DEFINE_PER_CPU(struct irq_work *, irq_work_list) = {
+	CALLBACK_TAIL,
+};
+
+int irq_work_queue(struct irq_work *entry, void (*func)(struct irq_work *))
+{
+	struct irq_work **head;
+
+	if (cmpxchg(&entry->next, NULL, CALLBACK_TAIL) != NULL)
+		return 0;
+
+	entry->func = func;
+
+	head = &get_cpu_var(irq_work_list);
+
+	do {
+		entry->next = *head;
+	} while (cmpxchg(head, entry->next, entry) != entry->next);
+
+	if (entry->next == CALLBACK_TAIL)
+		arch_self_ipi();
+
+	put_cpu_var(irq_work_list);
+	return 1;
+}
+
+void irq_work_run(void)
+{
+	struct irq_work *list;
+
+	list = xchg(&__get_cpu_var(irq_work_list), CALLBACK_TAIL);
+	while (list != CALLBACK_TAIL) {
+		struct irq_work *entry = list;
+
+		list = list->next;
+		entry->func(entry);
+
+		entry->next = NULL;
+		/*
+		 * matches the mb in cmpxchg() in irq_work_queue()
+		 */
+		smp_wmb();
+	}
+}
+
+static int irq_work_pending(struct irq_work *entry)
+{
+	/*
+	 * matches the wmb in irq_work_run
+	 */
+	smp_rmb();
+	return entry->next != NULL;
+}
+
+void irq_work_sync(struct irq_work *entry)
+{
+	WARN_ON_ONCE(irqs_disabled());
+
+	while (irq_work_pending(entry))
+		cpu_relax();
+}