make cancel_rearming_delayed_work() reliable

Thanks to Jarek Poplawski for the ideas and for spotting the bug in the
initial draft patch.

cancel_rearming_delayed_work() currently has many limitations, because it
requires that dwork always re-arms itself via queue_delayed_work().  So it
hangs forever if dwork doesn't do this, or cancel_rearming_delayed_work/
cancel_delayed_work was already called.  It uses flush_workqueue() in a
loop, so it can't be used if workqueue was freezed, and it is potentially
live- lockable on busy system if delay is small.

With this patch cancel_rearming_delayed_work() doesn't make any assumptions
about dwork, it can re-arm itself via queue_delayed_work(), or
queue_work(), or do nothing.

As a "side effect", cancel_work_sync() was changed to handle re-arming works
as well.

Disadvantages:

	- this patch adds wmb() to insert_work().

	- slowdowns the fast path (when del_timer() succeeds on entry) of
	  cancel_rearming_delayed_work(), because wait_on_work() is called
	  unconditionally. In that case, compared to the old version, we are
	  doing "unneeded" lock/unlock for each online CPU.

	  On the other hand, this means we don't need to use cancel_work_sync()
	  after cancel_rearming_delayed_work().

	- complicates the code (.text grows by 130 bytes).

[akpm@linux-foundation.org: fix speling]
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: David Chinner <dgc@sgi.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Gautham Shenoy <ego@in.ibm.com>
Acked-by: Jarek Poplawski <jarkao2@o2.pl>
Cc: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 91 insertions, 49 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 25cee1afe6fb..b976ed87dd37 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -120,6 +120,11 @@ static void insert_work(struct cpu_workqueue_struct *cwq,
                                 struct work_struct *work, int tail)
 {
         set_wq_data(work, cwq);
+        /*
+         * Ensure that we get the right work->data if we see the
+         * result of list_add() below, see try_to_grab_pending().
+         */
+        smp_wmb();
         if (tail)
                 list_add_tail(&work->entry, &cwq->worklist);
         else
@@ -383,7 +388,46 @@ void fastcall flush_workqueue(struct workqueue_struct *wq)
 }
 EXPORT_SYMBOL_GPL(flush_workqueue);
 
-static void wait_on_work(struct cpu_workqueue_struct *cwq,
+/*
+ * Upon a successful return, the caller "owns" WORK_STRUCT_PENDING bit,
+ * so this work can't be re-armed in any way.
+ */
+static int try_to_grab_pending(struct work_struct *work)
+{
+        struct cpu_workqueue_struct *cwq;
+        int ret = 0;
+
+        if (!test_and_set_bit(WORK_STRUCT_PENDING, work_data_bits(work)))
+                return 1;
+
+        /*
+         * The queueing is in progress, or it is already queued. Try to
+         * steal it from ->worklist without clearing WORK_STRUCT_PENDING.
+         */
+
+        cwq = get_wq_data(work);
+        if (!cwq)
+                return ret;
+
+        spin_lock_irq(&cwq->lock);
+        if (!list_empty(&work->entry)) {
+                /*
+                 * This work is queued, but perhaps we locked the wrong cwq.
+                 * In that case we must see the new value after rmb(), see
+                 * insert_work()->wmb().
+                 */
+                smp_rmb();
+                if (cwq == get_wq_data(work)) {
+                        list_del_init(&work->entry);
+                        ret = 1;
+                }
+        }
+        spin_unlock_irq(&cwq->lock);
+
+        return ret;
+}
+
+static void wait_on_cpu_work(struct cpu_workqueue_struct *cwq,
                                 struct work_struct *work)
 {
         struct wq_barrier barr;
@@ -400,20 +444,7 @@ static void wait_on_work(struct cpu_workqueue_struct *cwq,
                 wait_for_completion(&barr.done);
 }
 
-/**
- * cancel_work_sync - block until a work_struct's callback has terminated
- * @work: the work which is to be flushed
- *
- * cancel_work_sync() will attempt to cancel the work if it is queued. If the
- * work's callback appears to be running, cancel_work_sync() will block until
- * it has completed.
- *
- * cancel_work_sync() is designed to be used when the caller is tearing down
- * data structures which the callback function operates upon. It is expected
- * that, prior to calling cancel_work_sync(), the caller has arranged for the
- * work to not be requeued.
- */
-void cancel_work_sync(struct work_struct *work)
+static void wait_on_work(struct work_struct *work)
 {
         struct cpu_workqueue_struct *cwq;
         struct workqueue_struct *wq;
@@ -423,29 +454,62 @@ void cancel_work_sync(struct work_struct *work)
         might_sleep();
 
         cwq = get_wq_data(work);
-        /* Was it ever queued ? */
         if (!cwq)
                 return;
 
-        /*
-         * This work can't be re-queued, no need to re-check that
-         * get_wq_data() is still the same when we take cwq->lock.
-         */
-        spin_lock_irq(&cwq->lock);
-        list_del_init(&work->entry);
-        work_clear_pending(work);
-        spin_unlock_irq(&cwq->lock);
-
         wq = cwq->wq;
         cpu_map = wq_cpu_map(wq);
 
         for_each_cpu_mask(cpu, *cpu_map)
-                wait_on_work(per_cpu_ptr(wq->cpu_wq, cpu), work);
+                wait_on_cpu_work(per_cpu_ptr(wq->cpu_wq, cpu), work);
+}
+
+/**
+ * cancel_work_sync - block until a work_struct's callback has terminated
+ * @work: the work which is to be flushed
+ *
+ * cancel_work_sync() will cancel the work if it is queued. If the work's
+ * callback appears to be running, cancel_work_sync() will block until it
+ * has completed.
+ *
+ * It is possible to use this function if the work re-queues itself. It can
+ * cancel the work even if it migrates to another workqueue, however in that
+ * case it only guarantees that work->func() has completed on the last queued
+ * workqueue.
+ *
+ * cancel_work_sync(&delayed_work->work) should be used only if ->timer is not
+ * pending, otherwise it goes into a busy-wait loop until the timer expires.
+ *
+ * The caller must ensure that workqueue_struct on which this work was last
+ * queued can't be destroyed before this function returns.
+ */
+void cancel_work_sync(struct work_struct *work)
+{
+        while (!try_to_grab_pending(work))
+                cpu_relax();
+        wait_on_work(work);
+        work_clear_pending(work);
 }
 EXPORT_SYMBOL_GPL(cancel_work_sync);
 
+/**
+ * cancel_rearming_delayed_work - reliably kill off a delayed work.
+ * @dwork: the delayed work struct
+ *
+ * It is possible to use this function if @dwork rearms itself via queue_work()
+ * or queue_delayed_work(). See also the comment for cancel_work_sync().
+ */
+void cancel_rearming_delayed_work(struct delayed_work *dwork)
+{
+        while (!del_timer(&dwork->timer) &&
+               !try_to_grab_pending(&dwork->work))
+                cpu_relax();
+        wait_on_work(&dwork->work);
+        work_clear_pending(&dwork->work);
+}
+EXPORT_SYMBOL(cancel_rearming_delayed_work);
 
-static struct workqueue_struct *keventd_wq;
+static struct workqueue_struct *keventd_wq __read_mostly;
 
 /**
  * schedule_work - put work task in global workqueue
@@ -532,28 +596,6 @@ void flush_scheduled_work(void)
 EXPORT_SYMBOL(flush_scheduled_work);
 
 /**
- * cancel_rearming_delayed_work - kill off a delayed work whose handler rearms the delayed work.
- * @dwork: the delayed work struct
- *
- * Note that the work callback function may still be running on return from
- * cancel_delayed_work(). Run flush_workqueue() or cancel_work_sync() to wait
- * on it.
- */
-void cancel_rearming_delayed_work(struct delayed_work *dwork)
-{
-        struct cpu_workqueue_struct *cwq = get_wq_data(&dwork->work);
-
-        /* Was it ever queued ? */
-        if (cwq != NULL) {
-                struct workqueue_struct *wq = cwq->wq;
-
-                while (!cancel_delayed_work(dwork))
-                        flush_workqueue(wq);
-        }
-}
-EXPORT_SYMBOL(cancel_rearming_delayed_work);
-
-/**
  * execute_in_process_context - reliably execute the routine with user context
  * @fn:         the function to execute
  * @ew:         guaranteed storage for the execute work structure (must