linux-kernel - [RFC 3/9] workqueue: require CPU hotplug read exclusion for apply_workqueue

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Message-Id: <20190725212505.15055-4-daniel.m.jordan@oracle.com>
Date:   Thu, 25 Jul 2019 17:24:59 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Steffen Klassert <steffen.klassert@...unet.com>,
        Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Tejun Heo <tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        Daniel Jordan <daniel.m.jordan@...cle.com>
Subject: [RFC 3/9] workqueue: require CPU hotplug read exclusion for apply_workqueue_attrs

Change the calling convention for apply_workqueue_attrs to require CPU
hotplug read exclusion.

Avoids lockdep complaints about nested calls to get_online_cpus in a
future patch where padata calls apply_workqueue_attrs when changing
other CPU-hotplug-sensitive data structures with the CPU read lock
already held.

Signed-off-by: Daniel Jordan <daniel.m.jordan@...cle.com>
---
 kernel/workqueue.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index f53705ff3ff1..bc2e09a8ea61 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -4030,6 +4030,8 @@ static int apply_workqueue_attrs_locked(struct workqueue_struct *wq,
  *
  * Performs GFP_KERNEL allocations.
  *
+ * Assumes caller has CPU hotplug read exclusion, i.e. get_online_cpus().
+ *
  * Return: 0 on success and -errno on failure.
  */
 int apply_workqueue_attrs(struct workqueue_struct *wq,
@@ -4037,9 +4039,11 @@ int apply_workqueue_attrs(struct workqueue_struct *wq,
 {
 	int ret;
 
-	apply_wqattrs_lock();
+	lockdep_assert_cpus_held();
+
+	mutex_lock(&wq_pool_mutex);
 	ret = apply_workqueue_attrs_locked(wq, attrs);
-	apply_wqattrs_unlock();
+	mutex_unlock(&wq_pool_mutex);
 
 	return ret;
 }
@@ -4152,16 +4156,21 @@ static int alloc_and_link_pwqs(struct workqueue_struct *wq)
 			mutex_unlock(&wq->mutex);
 		}
 		return 0;
-	} else if (wq->flags & __WQ_ORDERED) {
+	}
+
+	get_online_cpus();
+	if (wq->flags & __WQ_ORDERED) {
 		ret = apply_workqueue_attrs(wq, ordered_wq_attrs[highpri]);
 		/* there should only be single pwq for ordering guarantee */
 		WARN(!ret && (wq->pwqs.next != &wq->dfl_pwq->pwqs_node ||
 			      wq->pwqs.prev != &wq->dfl_pwq->pwqs_node),
 		     "ordering guarantee broken for workqueue %s\n", wq->name);
-		return ret;
 	} else {
-		return apply_workqueue_attrs(wq, unbound_std_wq_attrs[highpri]);
+		ret = apply_workqueue_attrs(wq, unbound_std_wq_attrs[highpri]);
 	}
+	put_online_cpus();
+
+	return ret;
 }
 
 static int wq_clamp_max_active(int max_active, unsigned int flags,
-- 
2.22.0