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Subject[rfc: cpuops adv V1 4/8] irq_work: Use per cpu atomics instead of regular atomics
The irq work queue is a per cpu object and it is sufficient for
synchronization if per cpu atomics are used. Doing so simplifies
the code and reduces the overhead of the code.

Before:

christoph@linux-2.6$ size kernel/irq_work.o
text data bss dec hex filename
451 8 1 460 1cc kernel/irq_work.o

After:

christoph@linux-2.6$ size kernel/irq_work.o
text data bss dec hex filename
438 8 1 447 1bf kernel/irq_work.o

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Christoph Lameter <cl@linux.com>

---
kernel/irq_work.c | 18 +++++++++---------
1 file changed, 9 insertions(+), 9 deletions(-)
Index: linux-2.6/kernel/irq_work.c
===================================================================
--- linux-2.6.orig/kernel/irq_work.c 2010-12-02 12:16:54.000000000 -0600
+++ linux-2.6/kernel/irq_work.c 2010-12-02 12:18:26.000000000 -0600
@@ -77,21 +77,21 @@ void __weak arch_irq_work_raise(void)
*/
static void __irq_work_queue(struct irq_work *entry)
{
- struct irq_work **head, *next;
+ struct irq_work *next;

- head = &get_cpu_var(irq_work_list);
+ preempt_disable();

do {
- next = *head;
+ next = __this_cpu_read(irq_work_list);
/* Can assign non-atomic because we keep the flags set. */
entry->next = next_flags(next, IRQ_WORK_FLAGS);
- } while (cmpxchg(head, next, entry) != next);
+ } while (this_cpu_cmpxchg(irq_work_list, next, entry) != next);

/* The list was empty, raise self-interrupt to start processing. */
if (!irq_work_next(entry))
arch_irq_work_raise();

- put_cpu_var(irq_work_list);
+ preempt_enable();
}

/*
@@ -120,16 +120,16 @@ EXPORT_SYMBOL_GPL(irq_work_queue);
*/
void irq_work_run(void)
{
- struct irq_work *list, **head;
+ struct irq_work *list;

- head = &__get_cpu_var(irq_work_list);
- if (*head == NULL)
+ if (this_cpu_read(irq_work_list) == NULL)
return;

BUG_ON(!in_irq());
BUG_ON(!irqs_disabled());

- list = xchg(head, NULL);
+ list = this_cpu_xchg(irq_work_list, NULL);
+
while (list != NULL) {
struct irq_work *entry = list;



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