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>

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 9aedd9f..32f8e0d 100644 (file)
--- 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;
 }