workqueue: avoid recursion in run_workqueue()

1) lockdep will complain when run_workqueue() performs recursion.

2) The recursive implementation of run_workqueue() means that
   flush_workqueue() and its documentation are inconsistent.  This may
   hide deadlocks and other bugs.

3) The recursion in run_workqueue() will poison cwq->current_work, but
   flush_work() and __cancel_work_timer(), etcetera need a reliable
   cwq->current_work.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Eric Dumazet <dada1@cosmosbay.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 30 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 9aedd9fd825b..32f8e0d2bf5a 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -48,8 +48,6 @@ struct cpu_workqueue_struct {
 
         struct workqueue_struct *wq;
         struct task_struct *thread;
-
-        int run_depth;          /* Detect run_workqueue() recursion depth */
 } ____cacheline_aligned;
 
 /*
@@ -262,13 +260,6 @@ EXPORT_SYMBOL_GPL(queue_delayed_work_on);
 static void run_workqueue(struct cpu_workqueue_struct *cwq)
 {
         spin_lock_irq(&cwq->lock);
-        cwq->run_depth++;
-        if (cwq->run_depth > 3) {
-                /* morton gets to eat his hat */
-                printk("%s: recursion depth exceeded: %d\n",
-                        __func__, cwq->run_depth);
-                dump_stack();
-        }
         while (!list_empty(&cwq->worklist)) {
                 struct work_struct *work = list_entry(cwq->worklist.next,
                                                 struct work_struct, entry);
@@ -311,7 +302,6 @@ static void run_workqueue(struct cpu_workqueue_struct *cwq)
                 spin_lock_irq(&cwq->lock);
                 cwq->current_work = NULL;
         }
-        cwq->run_depth--;
         spin_unlock_irq(&cwq->lock);
 }
 
@@ -368,29 +358,20 @@ static void insert_wq_barrier(struct cpu_workqueue_struct *cwq,
 
 static int flush_cpu_workqueue(struct cpu_workqueue_struct *cwq)
 {
-        int active;
+        int active = 0;
+        struct wq_barrier barr;
 
-        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;
+        WARN_ON(cwq->thread == current);
 
-                active = 0;
-                spin_lock_irq(&cwq->lock);
-                if (!list_empty(&cwq->worklist) || cwq->current_work != NULL) {
-                        insert_wq_barrier(cwq, &barr, &cwq->worklist);
-                        active = 1;
-                }
-                spin_unlock_irq(&cwq->lock);
-
-                if (active)
-                        wait_for_completion(&barr.done);
+        spin_lock_irq(&cwq->lock);
+        if (!list_empty(&cwq->worklist) || cwq->current_work != NULL) {
+                insert_wq_barrier(cwq, &barr, &cwq->worklist);
+                active = 1;
         }
+        spin_unlock_irq(&cwq->lock);
+
+        if (active)
+                wait_for_completion(&barr.done);
 
         return active;
 }