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Date: Fri, 12 Jan 2007 02:12:44 +0300
From: Oleg Nesterov <oleg@...sign.ru>
To: Andrew Morton <akpm@...l.org>
Cc: Srivatsa Vaddagiri <vatsa@...ibm.com>,
"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
Gautham shenoy <ego@...ibm.com>, Ingo Molnar <mingo@...e.hu>,
David Howells <dhowells@...hat.com>,
Linus Torvalds <torvalds@...l.org>,
linux-kernel@...r.kernel.org
Subject: [PATCH 2/3] workqueue: fix flush_workqueue() vs CPU_DEAD race
(Andrew, please drop fix-flush_workqueue-vs-cpu_dead-race.patch)
Many thanks to Srivatsa Vaddagiri for the helpful discussion and for spotting
the bug in my previous attempt.
work->func() (and thus flush_workqueue()) must not use workqueue_mutex,
this leads to deadlock when CPU_DEAD does kthread_stop(). However without
this mutex held we can't detect CPU_DEAD in progress, which can move pending
works to another CPU while the dead one is not on cpu_online_map.
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
!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.
Also, add migrate_sequence incremented by take_over_work() under cwq->lock.
If take_over_work() happened before we checked this CPU, we should see the
new value after spin_unlock().
Further possible changes:
remove CPU_DEAD handling (along with take_over_work, migrate_sequence)
from workqueue.c. CPU_DEAD just sets cwq->please_exit_after_flush flag.
CPU_UP_PREPARE->create_workqueue_thread() clears this flag, and creates
the new thread if cwq->thread == NULL.
This way the workqueue/cpu-hotplug interaction is almost zero, workqueue_mutex
just protects "workqueues" list, CPU_LOCK_ACQUIRE/CPU_LOCK_RELEASE go away.
Signed-off-by: Oleg Nesterov <oleg@...sign.ru>
--- mm-6.20-rc3/kernel/workqueue.c~2_race 2007-01-11 21:38:12.000000000 +0300
+++ mm-6.20-rc3/kernel/workqueue.c 2007-01-11 22:22:58.000000000 +0300
@@ -64,6 +64,7 @@ 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);
@@ -421,13 +422,7 @@ 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;
@@ -439,11 +434,8 @@ static void flush_cpu_workqueue(struct c
}
spin_unlock_irq(&cwq->lock);
- if (active) {
- preempt_enable();
+ if (active)
wait_for_completion(&barr.done);
- preempt_disable();
- }
}
}
@@ -462,17 +454,21 @@ 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);
@@ -544,17 +540,21 @@ out:
}
EXPORT_SYMBOL_GPL(flush_work);
-static struct task_struct *create_workqueue_thread(struct workqueue_struct *wq,
- int cpu)
+static void init_cpu_workqueue(struct workqueue_struct *wq, int cpu)
{
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;
+ spin_lock_init(&cwq->lock);
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);
@@ -589,6 +589,7 @@ struct workqueue_struct *__create_workqu
mutex_lock(&workqueue_mutex);
if (singlethread) {
INIT_LIST_HEAD(&wq->list);
+ init_cpu_workqueue(wq, singlethread_cpu);
p = create_workqueue_thread(wq, singlethread_cpu);
if (!p)
destroy = 1;
@@ -596,7 +597,11 @@ struct workqueue_struct *__create_workqu
wake_up_process(p);
} else {
list_add(&wq->list, &workqueues);
- for_each_online_cpu(cpu) {
+ for_each_possible_cpu(cpu) {
+ init_cpu_workqueue(wq, cpu);
+ if (!cpu_online(cpu))
+ continue;
+
p = create_workqueue_thread(wq, cpu);
if (p) {
kthread_bind(p, cpu);
@@ -833,6 +838,7 @@ 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);
-
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