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Subject[PATCH wq/for-3.9 workqueue]: add delayed_work->wq to simplify reentrancy handling
From: Lai Jiangshan <laijs@cn.fujitsu.com>

To avoid executing the same work item from multiple CPUs concurrently,
a work_struct records the last pool it was on in its ->data so that,
on the next queueing, the pool can be queried to determine whether the
work item is still executing or not.

A delayed_work goes through timer before actually being queued on the
target workqueue and the timer needs to know the target workqueue and
CPU. This is currently achieved by modifying delayed_work->work.data
such that it points to the cwq which points to the target workqueue
and the last CPU the work item was on. __queue_delayed_work()
extracts the last CPU from delayed_work->work.data and then combines
it with the target workqueue to create new work.data.

The only thing this rather ugly hack achieves is encoding the target
workqueue into delayed_work->work.data without using a separate field,
which could be a trade off one can make; unfortunately, this entangles
work->data management between regular workqueue and delayed_work code
by setting cwq pointer before the work item is actually queued and
becomes a hindrance for further improvements of work->data handling.

This can be easily made sane by adding a target workqueue field to
delayed_work. While delayed_work is used widely in the kernel and
this does make it a bit larger (<5%), I think this is the right
trade-off especially given the prospect of much saner handling of
work->data which currently involves quite tricky memory barrier
dancing, and don't expect to see any measureable effect.

Add delayed_work->wq and drop the delayed_work->work.data overloading.

tj: Rewrote the description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
---
include/linux/workqueue.h | 3 +++
kernel/workqueue.c | 32 +++-----------------------------
2 files changed, 6 insertions(+), 29 deletions(-)

--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -109,6 +109,9 @@ struct work_struct {
struct delayed_work {
struct work_struct work;
struct timer_list timer;
+
+ /* target workqueue and CPU ->timer uses to queue ->work */
+ struct workqueue_struct *wq;
int cpu;
};

--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1339,10 +1339,9 @@ EXPORT_SYMBOL_GPL(queue_work);
void delayed_work_timer_fn(unsigned long __data)
{
struct delayed_work *dwork = (struct delayed_work *)__data;
- struct cpu_workqueue_struct *cwq = get_work_cwq(&dwork->work);

/* should have been called from irqsafe timer with irq already off */
- __queue_work(dwork->cpu, cwq->wq, &dwork->work);
+ __queue_work(dwork->cpu, dwork->wq, &dwork->work);
}
EXPORT_SYMBOL_GPL(delayed_work_timer_fn);

@@ -1351,7 +1350,6 @@ static void __queue_delayed_work(int cpu
{
struct timer_list *timer = &dwork->timer;
struct work_struct *work = &dwork->work;
- unsigned int lcpu;

WARN_ON_ONCE(timer->function != delayed_work_timer_fn ||
timer->data != (unsigned long)dwork);
@@ -1371,30 +1369,7 @@ static void __queue_delayed_work(int cpu

timer_stats_timer_set_start_info(&dwork->timer);

- /*
- * This stores cwq for the moment, for the timer_fn. Note that the
- * work's pool is preserved to allow reentrance detection for
- * delayed works.
- */
- if (!(wq->flags & WQ_UNBOUND)) {
- struct worker_pool *pool = get_work_pool(work);
-
- /*
- * If we cannot get the last pool from @work directly,
- * select the last CPU such that it avoids unnecessarily
- * triggering non-reentrancy check in __queue_work().
- */
- lcpu = cpu;
- if (pool)
- lcpu = pool->cpu;
- if (lcpu == WORK_CPU_UNBOUND)
- lcpu = raw_smp_processor_id();
- } else {
- lcpu = WORK_CPU_UNBOUND;
- }
-
- set_work_cwq(work, get_cwq(lcpu, wq), 0);
-
+ dwork->wq = wq;
dwork->cpu = cpu;
timer->expires = jiffies + delay;

@@ -2944,8 +2919,7 @@ bool flush_delayed_work(struct delayed_w
{
local_irq_disable();
if (del_timer_sync(&dwork->timer))
- __queue_work(dwork->cpu,
- get_work_cwq(&dwork->work)->wq, &dwork->work);
+ __queue_work(dwork->cpu, dwork->wq, &dwork->work);
local_irq_enable();
return flush_work(&dwork->work);
}

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