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Message-ID: <20070518203429.GA325@tv-sign.ru>
Date: Sat, 19 May 2007 00:34:29 +0400
From: Oleg Nesterov <oleg@...sign.ru>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Zilvinas Valinskas <zilvinas@...ibox.com>,
Gautham R Shenoy <ego@...ibm.com>,
Srivatsa Vaddagiri <vatsa@...ibm.com>,
"Rafael J. Wysocki" <rjw@...k.pl>, linux-kernel@...r.kernel.org
Subject: [PATCH] simplify cleanup_workqueue_thread()
cleanup_workqueue_thread() and cwq_should_stop() are overcomplicated. Convert
the code to use kthread_should_stop/kthread_stop as was suggested by Gautham
and Srivatsa.
In particular this patch removes the (unlikely) busy-wait loop from the exit
path, it was a temporary and ugly kludge (if not a bug).
Note: the current code was designed to solve another old problem: work->func
can't share locks with hotplug callbacks. I think this could be done, see
http://marc.info/?l=linux-kernel&m=116905366428633
but this needs some more complications to preserve CPU affinity of cwq->thread
during cpu_up(). A freezer-based hotplug looks more appealing.
Signed-off-by: Oleg Nesterov <oleg@...sign.ru>
--- OLD/kernel/workqueue.c~1_KILL_CRAP 2007-05-13 15:19:54.000000000 +0400
+++ OLD/kernel/workqueue.c 2007-05-18 00:12:05.000000000 +0400
@@ -47,7 +47,6 @@ struct cpu_workqueue_struct {
struct workqueue_struct *wq;
struct task_struct *thread;
- int should_stop;
int run_depth; /* Detect run_workqueue() recursion depth */
} ____cacheline_aligned;
@@ -71,7 +70,13 @@ static LIST_HEAD(workqueues);
static int singlethread_cpu __read_mostly;
static cpumask_t cpu_singlethread_map __read_mostly;
-/* optimization, we could use cpu_possible_map */
+/*
+ * _cpu_down() first removes CPU from cpu_online_map, then CPU_DEAD
+ * flushes cwq->worklist. This means that flush_workqueue/wait_on_work
+ * which comes in between can't use for_each_online_cpu(). We could
+ * use cpu_possible_map, the cpumask below is more a documentation
+ * than optimization.
+ */
static cpumask_t cpu_populated_map __read_mostly;
/* If it's single threaded, it isn't in the list of workqueues. */
@@ -272,24 +277,6 @@ static void run_workqueue(struct cpu_wor
spin_unlock_irq(&cwq->lock);
}
-/*
- * NOTE: the caller must not touch *cwq if this func returns true
- */
-static int cwq_should_stop(struct cpu_workqueue_struct *cwq)
-{
- int should_stop = cwq->should_stop;
-
- if (unlikely(should_stop)) {
- spin_lock_irq(&cwq->lock);
- should_stop = cwq->should_stop && list_empty(&cwq->worklist);
- if (should_stop)
- cwq->thread = NULL;
- spin_unlock_irq(&cwq->lock);
- }
-
- return should_stop;
-}
-
static int worker_thread(void *__cwq)
{
struct cpu_workqueue_struct *cwq = __cwq;
@@ -302,14 +289,15 @@ static int worker_thread(void *__cwq)
for (;;) {
prepare_to_wait(&cwq->more_work, &wait, TASK_INTERRUPTIBLE);
- if (!freezing(current) && !cwq->should_stop
- && list_empty(&cwq->worklist))
+ if (!freezing(current) &&
+ !kthread_should_stop() &&
+ list_empty(&cwq->worklist))
schedule();
finish_wait(&cwq->more_work, &wait);
try_to_freeze();
- if (cwq_should_stop(cwq))
+ if (kthread_should_stop())
break;
run_workqueue(cwq);
@@ -340,7 +328,7 @@ static void insert_wq_barrier(struct cpu
insert_work(cwq, &barr->work, tail);
}
-static void flush_cpu_workqueue(struct cpu_workqueue_struct *cwq)
+static int flush_cpu_workqueue(struct cpu_workqueue_struct *cwq)
{
if (cwq->thread == current) {
/*
@@ -348,6 +336,7 @@ static void flush_cpu_workqueue(struct c
* it by hand rather than deadlocking.
*/
run_workqueue(cwq);
+ return 1;
} else {
struct wq_barrier barr;
int active = 0;
@@ -361,6 +350,8 @@ static void flush_cpu_workqueue(struct c
if (active)
wait_for_completion(&barr.done);
+
+ return active;
}
}
@@ -674,7 +665,6 @@ static int create_workqueue_thread(struc
return PTR_ERR(p);
cwq->thread = p;
- cwq->should_stop = 0;
return 0;
}
@@ -740,29 +730,27 @@ EXPORT_SYMBOL_GPL(__create_workqueue);
static void cleanup_workqueue_thread(struct cpu_workqueue_struct *cwq, int cpu)
{
- struct wq_barrier barr;
- int alive = 0;
-
- spin_lock_irq(&cwq->lock);
- if (cwq->thread != NULL) {
- insert_wq_barrier(cwq, &barr, 1);
- cwq->should_stop = 1;
- alive = 1;
- }
- spin_unlock_irq(&cwq->lock);
+ /*
+ * Our caller is either destroy_workqueue() or CPU_DEAD,
+ * workqueue_mutex protects cwq->thread
+ */
+ if (cwq->thread == NULL)
+ return;
- if (alive) {
- wait_for_completion(&barr.done);
+ /*
+ * If the caller is CPU_DEAD the single flush_cpu_workqueue()
+ * is not enough, a concurrent flush_workqueue() can insert a
+ * barrier after us.
+ * When ->worklist becomes empty it is safe to exit because no
+ * more work_structs can be queued on this cwq: flush_workqueue
+ * checks list_empty(), and a "normal" queue_work() can't use
+ * a dead CPU.
+ */
+ while (flush_cpu_workqueue(cwq))
+ ;
- while (unlikely(cwq->thread != NULL))
- cpu_relax();
- /*
- * Wait until cwq->thread unlocks cwq->lock,
- * it won't touch *cwq after that.
- */
- smp_rmb();
- spin_unlock_wait(&cwq->lock);
- }
+ kthread_stop(cwq->thread);
+ cwq->thread = NULL;
}
/**
-
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