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Date: Tue, 24 Apr 2007 22:55:37 +0400
From: Oleg Nesterov <oleg@...sign.ru>
To: Jarek Poplawski <jarkao2@...pl>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
David Howells <dhowells@...hat.com>
Subject: Re: Fw: [PATCH -mm] workqueue: debug possible endless loop in cancel_rearming_delayed_work
On 04/24, Jarek Poplawski wrote:
>
> This looks fine. Of course, it requires to remove some debugging
> currently done with _PENDING flag
For example?
> and it's hard to estimate this
> all before you do more, but it should be more foreseeable than
> current way. But the races with _PENDING could be really "funny"
> without locking it everywhere.
Please see the patch below. Do you see any problems? I'll send it
when I have time to re-read the code and write some tests. I still
hope we can find a way to avoid the change in run_workqueue()...
Note that cancel_rearming_delayed_work() now can handle the works
which re-arm itself via queue_work(), not only queue_delayed_work().
Note also we can change cancel_work_sync(), so it can deal with the
self rearming work_structs.
> BTW - are a few locks more a real
> problem, while serving a "sleeping" path? And I don't think there
> is any reason to hurry...
Sorry, could you clarify what you mean?
> > > Yes, but currently you cannot to behave like this e.g. with
> > > "rearming" work.
> >
> > Why?
>
> OK, it's not impossible, but needs some bothering: if I simply
> set some flag and my work function exits before rearming -
> cancel_rearming_delayed_work can loop.
Yes sure. I meant "after we fix the problems you pointed out".
Oleg.
--- OLD/kernel/workqueue.c~1_CRDW 2007-04-13 17:43:23.000000000 +0400
+++ OLD/kernel/workqueue.c 2007-04-24 22:41:15.000000000 +0400
@@ -242,11 +242,11 @@ static void run_workqueue(struct cpu_wor
work_func_t f = work->func;
cwq->current_work = work;
- list_del_init(cwq->worklist.next);
+ list_del_init(&work->entry);
+ work_clear_pending(work);
spin_unlock_irq(&cwq->lock);
BUG_ON(get_wq_data(work) != cwq);
- work_clear_pending(work);
f(work);
if (unlikely(in_atomic() || lockdep_depth(current) > 0)) {
@@ -398,6 +398,16 @@ static void wait_on_work(struct cpu_work
wait_for_completion(&barr.done);
}
+static void needs_a_good_name(struct workqueue_struct *wq,
+ struct work_struct *work)
+{
+ const cpumask_t *cpu_map = wq_cpu_map(wq);
+ int cpu;
+
+ for_each_cpu_mask(cpu, *cpu_map)
+ wait_on_work(per_cpu_ptr(wq->cpu_wq, cpu), work);
+}
+
/**
* cancel_work_sync - block until a work_struct's callback has terminated
* @work: the work which is to be flushed
@@ -414,9 +424,6 @@ static void wait_on_work(struct cpu_work
void cancel_work_sync(struct work_struct *work)
{
struct cpu_workqueue_struct *cwq;
- struct workqueue_struct *wq;
- const cpumask_t *cpu_map;
- int cpu;
might_sleep();
@@ -434,15 +441,10 @@ void cancel_work_sync(struct work_struct
work_clear_pending(work);
spin_unlock_irq(&cwq->lock);
- wq = cwq->wq;
- cpu_map = wq_cpu_map(wq);
-
- for_each_cpu_mask(cpu, *cpu_map)
- wait_on_work(per_cpu_ptr(wq->cpu_wq, cpu), work);
+ needs_a_good_name(cwq->wq, work);
}
EXPORT_SYMBOL_GPL(cancel_work_sync);
-
static struct workqueue_struct *keventd_wq;
/**
@@ -532,22 +534,34 @@ EXPORT_SYMBOL(flush_scheduled_work);
/**
* cancel_rearming_delayed_work - kill off a delayed work whose handler rearms the delayed work.
* @dwork: the delayed work struct
- *
- * Note that the work callback function may still be running on return from
- * cancel_delayed_work(). Run flush_workqueue() or cancel_work_sync() to wait
- * on it.
*/
void cancel_rearming_delayed_work(struct delayed_work *dwork)
{
- struct cpu_workqueue_struct *cwq = get_wq_data(&dwork->work);
-
- /* Was it ever queued ? */
- if (cwq != NULL) {
- struct workqueue_struct *wq = cwq->wq;
-
- while (!cancel_delayed_work(dwork))
- flush_workqueue(wq);
- }
+ struct work_struct *work = &dwork->work;
+ struct cpu_workqueue_struct *cwq = get_wq_data(work);
+ int retry;
+
+ if (!cwq)
+ return;
+
+ do {
+ retry = 1;
+ spin_lock_irq(&cwq->lock);
+ /* CPU_DEAD in progress may change cwq */
+ if (likely(cwq == get_wq_data(work))) {
+ list_del_init(&work->entry);
+ __set_bit(WORK_STRUCT_PENDING, work_data_bits(work));
+ retry = try_to_del_timer_sync(&dwork->timer) < 0;
+ }
+ spin_unlock_irq(&cwq->lock);
+ } while (unlikely(retry));
+
+ /*
+ * Nobody can clear WORK_STRUCT_PENDING. This means that the
+ * work can't be re-queued and the timer can't be re-started.
+ */
+ needs_a_good_name(cwq->wq, work);
+ work_clear_pending(work);
}
EXPORT_SYMBOL(cancel_rearming_delayed_work);
-
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