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Message-ID: <20070107215103.GA7960@tv-sign.ru>
Date: Mon, 8 Jan 2007 00:51:03 +0300
From: Oleg Nesterov <oleg@...sign.ru>
To: Andrew Morton <akpm@...l.org>
Cc: vatsa@...ibm.com, David Howells <dhowells@...hat.com>,
Christoph Hellwig <hch@...radead.org>,
Ingo Molnar <mingo@...e.hu>,
Linus Torvalds <torvalds@...l.org>,
linux-kernel@...r.kernel.org, Gautham shenoy <ego@...ibm.com>
Subject: Re: [PATCH] fix-flush_workqueue-vs-cpu_dead-race-update
On 01/07, Andrew Morton wrote:
>
> Plus flush_workqueue() is on the way out. We're slowly edging towards a
> working cancel_work() which will only block if the work which you're trying
> to cancel is presently running. With that, pretty much all the
> flush_workqueue() calls go away, and all these accidental rarely-occurring
> deadlocks go away too.
So. If we can forget about the race we have - fine. Otherwise, how about the
patch below? It is untested and needs a review. I can't suggest any simpler
now.
Change flush_workqueue() to use for_each_possible_cpu(). This means that
flush_cpu_workqueue() may hit CPU which is already dead. However in that
case
if (!list_empty(&cwq->worklist) || cwq->current_work != NULL)
means that CPU_DEAD in progress, it will do kthread_stop() + take_over_work()
so we can proceed and insert a barrier. We hold cwq->lock, so we are safe.
This patch replaces fix-flush_workqueue-vs-cpu_dead-race.patch which was
broken by switching to preempt_disable (now we don't need locking at all).
Note that migrate_sequence (was hotplug_sequence) is incremented under
cwq->lock. Since flush_workqueue does lock/unlock of cwq->lock on all CPUs,
it must see the new value if take_over_work() happened before we checked
this cwq, and this is the case we should worry about: otherwise we added
a barrier.
Srivatsa?
--- mm-6.20-rc3/kernel/workqueue.c~2_race 2007-01-08 00:07:07.000000000 +0300
+++ mm-6.20-rc3/kernel/workqueue.c 2007-01-08 00:28:55.000000000 +0300
@@ -65,6 +65,7 @@ struct workqueue_struct {
/* All the per-cpu workqueues on the system, for hotplug cpu to add/remove
threads to each one as cpus come/go. */
+static long migrate_sequence __read_mostly;
static DEFINE_MUTEX(workqueue_mutex);
static LIST_HEAD(workqueues);
@@ -422,13 +423,7 @@ static void flush_cpu_workqueue(struct c
* Probably keventd trying to flush its own queue. So simply run
* it by hand rather than deadlocking.
*/
- preempt_enable();
- /*
- * We can still touch *cwq here because we are keventd, and
- * hot-unplug will be waiting us to exit.
- */
run_workqueue(cwq);
- preempt_disable();
} else {
struct wq_barrier barr;
int active = 0;
@@ -441,9 +436,7 @@ static void flush_cpu_workqueue(struct c
spin_unlock_irq(&cwq->lock);
if (active) {
- preempt_enable();
wait_for_completion(&barr.done);
- preempt_disable();
}
}
}
@@ -463,17 +456,21 @@ static void flush_cpu_workqueue(struct c
*/
void fastcall flush_workqueue(struct workqueue_struct *wq)
{
- preempt_disable(); /* CPU hotplug */
if (is_single_threaded(wq)) {
/* Always use first cpu's area. */
flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, singlethread_cpu));
} else {
+ long sequence;
int cpu;
+again:
+ sequence = migrate_sequence;
- for_each_online_cpu(cpu)
+ for_each_possible_cpu(cpu)
flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, cpu));
+
+ if (unlikely(sequence != migrate_sequence))
+ goto again;
}
- preempt_enable();
}
EXPORT_SYMBOL_GPL(flush_workqueue);
@@ -545,18 +542,22 @@ out:
}
EXPORT_SYMBOL_GPL(flush_work);
-static struct task_struct *create_workqueue_thread(struct workqueue_struct *wq,
- int cpu, int freezeable)
+static void init_cpu_workqueue(struct workqueue_struct *wq,
+ struct cpu_workqueue_struct *cwq, int freezeable)
{
- struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);
- struct task_struct *p;
-
spin_lock_init(&cwq->lock);
cwq->wq = wq;
cwq->thread = NULL;
cwq->freezeable = freezeable;
INIT_LIST_HEAD(&cwq->worklist);
init_waitqueue_head(&cwq->more_work);
+}
+
+static struct task_struct *create_workqueue_thread(struct workqueue_struct *wq,
+ int cpu)
+{
+ struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);
+ struct task_struct *p;
if (is_single_threaded(wq))
p = kthread_create(worker_thread, cwq, "%s", wq->name);
@@ -589,15 +590,20 @@ struct workqueue_struct *__create_workqu
mutex_lock(&workqueue_mutex);
if (singlethread) {
INIT_LIST_HEAD(&wq->list);
- p = create_workqueue_thread(wq, singlethread_cpu, freezeable);
+ init_cpu_workqueue(wq, per_cpu_ptr(wq->cpu_wq, singlethread_cpu),
+ freezeable);
+ p = create_workqueue_thread(wq, singlethread_cpu);
if (!p)
destroy = 1;
else
wake_up_process(p);
} else {
list_add(&wq->list, &workqueues);
+ for_each_possible_cpu(cpu)
+ init_cpu_workqueue(wq, per_cpu_ptr(wq->cpu_wq, cpu),
+ freezeable);
for_each_online_cpu(cpu) {
- p = create_workqueue_thread(wq, cpu, freezeable);
+ p = create_workqueue_thread(wq, cpu);
if (p) {
kthread_bind(p, cpu);
wake_up_process(p);
@@ -833,6 +839,7 @@ static void take_over_work(struct workqu
spin_lock_irq(&cwq->lock);
list_replace_init(&cwq->worklist, &list);
+ migrate_sequence++;
while (!list_empty(&list)) {
printk("Taking work for %s\n", wq->name);
@@ -859,7 +866,7 @@ static int __devinit workqueue_cpu_callb
case CPU_UP_PREPARE:
/* Create a new workqueue thread for it. */
list_for_each_entry(wq, &workqueues, list) {
- if (!create_workqueue_thread(wq, hotcpu, 0)) {
+ if (!create_workqueue_thread(wq, hotcpu)) {
printk("workqueue for %i failed\n", hotcpu);
return NOTIFY_BAD;
}
-
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