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.

[akpm@linux-foundation.org: make it more tolerant of gcc borkenness]
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Zilvinas Valinskas <zilvinas@wilibox.com>
Cc: Gautham R Shenoy <ego@in.ibm.com>
Cc: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 37 insertions, 47 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index fb56fedd5c02..3bebf73be976 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -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_workqueue_struct *cwq)
         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);
@@ -340,18 +328,21 @@ static void insert_wq_barrier(struct cpu_workqueue_struct *cwq,
         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)
 {
+        int active;
+
         if (cwq->thread == current) {
                 /*
                  * Probably keventd trying to flush its own queue. So simply run
                  * it by hand rather than deadlocking.
                  */
                 run_workqueue(cwq);
+                active = 1;
         } else {
                 struct wq_barrier barr;
-                int active = 0;
 
+                active = 0;
                 spin_lock_irq(&cwq->lock);
                 if (!list_empty(&cwq->worklist) || cwq->current_work != NULL) {
                         insert_wq_barrier(cwq, &barr, 1);
@@ -362,6 +353,8 @@ static void flush_cpu_workqueue(struct cpu_workqueue_struct *cwq)
                 if (active)
                         wait_for_completion(&barr.done);
         }
+
+        return active;
 }
 
 /**
@@ -674,7 +667,6 @@ static int create_workqueue_thread(struct cpu_workqueue_struct *cwq, int cpu)
                 return PTR_ERR(p);
 
         cwq->thread = p;
-        cwq->should_stop = 0;
 
         return 0;
 }
@@ -740,29 +732,27 @@ 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;
 }
 
 /**