[PATCH 5/7 V2] workqueue: reuse the current default pwq when its attrs unchanged

[Lai Jiangshan <laijs@cn.fujitsu.com>]

When apply_wqattrs_prepare() is called, it is possible that the default
pwq is unaffected.  It is always true that only the NUMA affinity is being
changed and sometimes true that the low level cpumask is being changed.

So we try to reuse the current default pwq when its attrs unchanged.

After this change, "ctx->dfl_pwq->refcnt++" could be dangerous
when ctx->dfl_pwq is being reused, so we use get_pwq_unlocked() instead.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
 kernel/workqueue.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 24e5fe4..61f8ace 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -3587,7 +3587,10 @@ apply_wqattrs_prepare(struct workqueue_struct *wq,
 	 * the default pwq covering whole @attrs->cpumask.  Always create
 	 * it even if we don't use it immediately.
 	 */
-	ctx->dfl_pwq = alloc_unbound_pwq(wq, new_attrs);
+	if (wq->dfl_pwq && wqattrs_equal(new_attrs, wq->dfl_pwq->pool->attrs))
+		ctx->dfl_pwq = get_pwq_unlocked(wq->dfl_pwq);
+	else
+		ctx->dfl_pwq = alloc_unbound_pwq(wq, new_attrs);
 	if (!ctx->dfl_pwq)
 		goto out_free;
 
@@ -3602,8 +3605,7 @@ apply_wqattrs_prepare(struct workqueue_struct *wq,
 				goto out_free;
 			ctx->pwq_tbl[node] = pwq;
 		} else {
-			ctx->dfl_pwq->refcnt++;
-			ctx->pwq_tbl[node] = ctx->dfl_pwq;
+			ctx->pwq_tbl[node] = get_pwq_unlocked(ctx->dfl_pwq);
 		}
 	}
 
-- 
2.1.0