[PATCH 05/13] workqueue: change queued detection and remove *mb()s

[Lai Jiangshan]

Now, we has this invariant:
        CWQ bit is set and the cwq->pool == pool
        <==> the work is queued on the pool

So we can simplify the work-queued-detection-and-lock and remove *mb()s.

(Although rmb()/wmb() is nop in x86, but it is very slow in other arch.)

Signed-off-by: Lai Jiangshan <la...@cn.fujitsu.com>
---
 kernel/workqueue.c |   43 +++++++++++++++++++------------------------
 1 files changed, 19 insertions(+), 24 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 50d3dd5..b7cfaa1 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1067,6 +1067,7 @@ static int try_to_grab_pending(struct work_struct *work, 
bool is_dwork,
                               unsigned long *flags)
 {
        struct worker_pool *pool;
+       struct cpu_workqueue_struct *cwq;
 
        local_irq_save(*flags);
 
@@ -1096,14 +1097,20 @@ static int try_to_grab_pending(struct work_struct 
*work, bool is_dwork,
                goto fail;
 
        spin_lock(&pool->lock);
-       if (!list_empty(&work->entry)) {
-               /*
-                * This work is queued, but perhaps we locked the wrong
-                * pool.  In that case we must see the new value after
-                * rmb(), see insert_work()->wmb().
-                */
-               smp_rmb();
-               if (pool == get_work_pool(work)) {
+       /*
+        * The CWQ bit is set/cleared only when we do enqueue/dequeue the work
+        * When a work is enqueued(insert_work()) to a pool:
+        *      we set cwq(CWQ bit) with pool->lock held
+        * when a work is dequeued(process_one_work(),try_to_grab_pending()):
+        *      we clear cwq(CWQ bit) with pool->lock held
+        *
+        * So when if the pool->lock is held, we can determine:
+        *      CWQ bit is set and the cwq->pool == pool
+        *      <==> the work is queued on the pool
+        */
+       cwq = get_work_cwq(work);
+       if (cwq) {
+               if (pool == cwq->pool) {
                        debug_work_deactivate(work);
 
                        /*
@@ -1156,13 +1163,6 @@ static void insert_work(struct cpu_workqueue_struct *cwq,
 
        /* we own @work, set data and link */
        set_work_cwq(work, cwq, extra_flags);
-
-       /*
-        * Ensure that we get the right work->data if we see the
-        * result of list_add() below, see try_to_grab_pending().
-        */
-       smp_wmb();
-
        list_add_tail(&work->entry, head);
 
        /*
@@ -2796,15 +2796,10 @@ static bool start_flush_work(struct work_struct *work, 
struct wq_barrier *barr)
                return false;
 
        spin_lock_irq(&pool->lock);
-       if (!list_empty(&work->entry)) {
-               /*
-                * See the comment near try_to_grab_pending()->smp_rmb().
-                * If it was re-queued to a different pool under us, we
-                * are not going to wait.
-                */
-               smp_rmb();
-               cwq = get_work_cwq(work);
-               if (unlikely(!cwq || pool != cwq->pool))
+       /* See the comment near try_to_grab_pending() with the same code */
+       cwq = get_work_cwq(work);
+       if (cwq) {
+               if (unlikely(pool != cwq->pool))
                        goto already_gone;
        } else {
                worker = find_worker_executing_work(pool, work);
-- 
1.7.7.6

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