linux-kernel - [PATCH 3/3] lockdep: Remove unnecessary acquisitions wrt workqueue flush

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Date:   Tue,  5 Sep 2017 11:29:14 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     tj@...nel.org, johannes.berg@...el.com, peterz@...radead.org,
        mingo@...nel.org
Cc:     tglx@...utronix.de, oleg@...hat.com, david@...morbit.com,
        linux-kernel@...r.kernel.org, kernel-team@....com
Subject: [PATCH 3/3] lockdep: Remove unnecessary acquisitions wrt workqueue flush

Workqueue added manual acquisitions to catch deadlock cases. Now
crossrelease was introduced, some of those are redundant because
crossrelease-enabled wait_for_completeion() also does it. Removed it.

Also, lock_map_acquire() in process_one_work() is too strong for
that purpose. lock_map_acquire_might() is enough. Replaced it.

Signed-off-by: Byungchul Park <byungchul.park@....com>
---
 kernel/workqueue.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index ab3c0dc..8b728d1 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2091,8 +2091,8 @@ static void process_one_work(struct worker *worker, struct work_struct *work)
 
 	spin_unlock_irq(&pool->lock);
 
-	lock_map_acquire(&pwq->wq->lockdep_map);
-	lock_map_acquire(&lockdep_map);
+	lock_map_acquire_might(&pwq->wq->lockdep_map);
+	lock_map_acquire_might(&lockdep_map);
 	/*
 	 * Strictly speaking we should mark the invariant state without holding
 	 * any locks, that is, before these two lock_map_acquire()'s.
@@ -2504,7 +2504,7 @@ static void insert_wq_barrier(struct pool_workqueue *pwq,
 	 */
 	lockdep_init_map_crosslock((struct lockdep_map *)&barr->done.map,
 				   "(complete)wq_barr::done",
-				   target->lockdep_map.key, 1);
+				   target->lockdep_map.key, 0);
 	__init_completion(&barr->done);
 	barr->task = current;
 
@@ -2611,16 +2611,17 @@ void flush_workqueue(struct workqueue_struct *wq)
 	struct wq_flusher this_flusher = {
 		.list = LIST_HEAD_INIT(this_flusher.list),
 		.flush_color = -1,
-		.done = COMPLETION_INITIALIZER_ONSTACK(this_flusher.done),
 	};
 	int next_color;
 
+	lockdep_init_map_crosslock((struct lockdep_map *)&this_flusher.done.map,
+				   "(complete)wq_flusher::done",
+				   wq->lockdep_map.key, 0);
+	__init_completion(&this_flusher.done);
+
 	if (WARN_ON(!wq_online))
 		return;
 
-	lock_map_acquire(&wq->lockdep_map);
-	lock_map_release(&wq->lockdep_map);
-
 	mutex_lock(&wq->mutex);
 
 	/*
@@ -2883,9 +2884,6 @@ bool flush_work(struct work_struct *work)
 	if (WARN_ON(!wq_online))
 		return false;
 
-	lock_map_acquire(&work->lockdep_map);
-	lock_map_release(&work->lockdep_map);
-
 	if (start_flush_work(work, &barr)) {
 		wait_for_completion(&barr.done);
 		destroy_work_on_stack(&barr.work);
-- 
1.9.1