workqueue: Use a kthread_worker to release pool_workqueues

pool_workqueue release path is currently bounced to system_wq; however, this
is a bit tricky because this bouncing occurs while holding a pool lock and
thus has risk of causing a A-A deadlock. This is currently addressed by the
fact that only unbound workqueues use this bouncing path and system_wq is a
per-cpu workqueue.

While this works, it's brittle and requires a work-around like setting the
lockdep subclass for the lock of unbound pools. Besides, future changes will
use the bouncing path for per-cpu workqueues too making the current approach
unusable.

Let's just use a dedicated kthread_worker to untangle the dependency. This
is just one more kthread for all workqueues and makes the pwq release logic
simpler and more robust.

Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 4347f49..01bf22c 100644 (file)
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -257,12 +257,12 @@ struct pool_workqueue {
        u64                     stats[PWQ_NR_STATS];
 
        /*
-        * Release of unbound pwq is punted to system_wq.  See put_pwq()
-        * and pwq_unbound_release_workfn() for details.  pool_workqueue
-        * itself is also RCU protected so that the first pwq can be
-        * determined without grabbing wq->mutex.
+        * Release of unbound pwq is punted to a kthread_worker. See put_pwq()
+        * and pwq_unbound_release_workfn() for details. pool_workqueue itself
+        * is also RCU protected so that the first pwq can be determined without
+        * grabbing wq->mutex.
         */
-       struct work_struct      unbound_release_work;
+       struct kthread_work     unbound_release_work;
        struct rcu_head         rcu;
 } __aligned(1 << WORK_STRUCT_FLAG_BITS);
 
@@ -395,6 +395,13 @@ static struct workqueue_attrs *unbound_std_wq_attrs[NR_STD_WORKER_POOLS];
 /* I: attributes used when instantiating ordered pools on demand */
 static struct workqueue_attrs *ordered_wq_attrs[NR_STD_WORKER_POOLS];
 
+/*
+ * I: kthread_worker to release pwq's. pwq release needs to be bounced to a
+ * process context while holding a pool lock. Bounce to a dedicated kthread
+ * worker to avoid A-A deadlocks.
+ */
+static struct kthread_worker *pwq_release_worker;
+
 struct workqueue_struct *system_wq __read_mostly;
 EXPORT_SYMBOL(system_wq);
 struct workqueue_struct *system_highpri_wq __read_mostly;
@@ -1366,14 +1373,10 @@ static void put_pwq(struct pool_workqueue *pwq)
        if (WARN_ON_ONCE(!(pwq->wq->flags & WQ_UNBOUND)))
                return;
        /*
-        * @pwq can't be released under pool->lock, bounce to
-        * pwq_unbound_release_workfn().  This never recurses on the same
-        * pool->lock as this path is taken only for unbound workqueues and
-        * the release work item is scheduled on a per-cpu workqueue.  To
-        * avoid lockdep warning, unbound pool->locks are given lockdep
-        * subclass of 1 in get_unbound_pool().
+        * @pwq can't be released under pool->lock, bounce to a dedicated
+        * kthread_worker to avoid A-A deadlocks.
         */
-       schedule_work(&pwq->unbound_release_work);
+       kthread_queue_work(pwq_release_worker, &pwq->unbound_release_work);
 }
 
 /**
@@ -3965,7 +3968,6 @@ static struct worker_pool *get_unbound_pool(const struct workqueue_attrs *attrs)
        if (!pool || init_worker_pool(pool) < 0)
                goto fail;
 
-       lockdep_set_subclass(&pool->lock, 1);   /* see put_pwq() */
        copy_workqueue_attrs(pool->attrs, attrs);
        pool->node = target_node;
 
@@ -3999,10 +4001,10 @@ static void rcu_free_pwq(struct rcu_head *rcu)
 }
 
 /*
- * Scheduled on system_wq by put_pwq() when an unbound pwq hits zero refcnt
- * and needs to be destroyed.
+ * Scheduled on pwq_release_worker by put_pwq() when an unbound pwq hits zero
+ * refcnt and needs to be destroyed.
  */
-static void pwq_unbound_release_workfn(struct work_struct *work)
+static void pwq_unbound_release_workfn(struct kthread_work *work)
 {
        struct pool_workqueue *pwq = container_of(work, struct pool_workqueue,
                                                  unbound_release_work);
@@ -4110,7 +4112,8 @@ static void init_pwq(struct pool_workqueue *pwq, struct workqueue_struct *wq,
        INIT_LIST_HEAD(&pwq->inactive_works);
        INIT_LIST_HEAD(&pwq->pwqs_node);
        INIT_LIST_HEAD(&pwq->mayday_node);
-       INIT_WORK(&pwq->unbound_release_work, pwq_unbound_release_workfn);
+       kthread_init_work(&pwq->unbound_release_work,
+                         pwq_unbound_release_workfn);
 }
 
 /* sync @pwq with the current state of its associated wq and link it */
@@ -6433,6 +6436,9 @@ static void __init wq_cpu_intensive_thresh_init(void)
        if (wq_cpu_intensive_thresh_us != ULONG_MAX)
                return;
 
+       pwq_release_worker = kthread_create_worker(0, "pool_workqueue_release");
+       BUG_ON(IS_ERR(pwq_release_worker));
+
        /*
         * The default of 10ms is derived from the fact that most modern (as of
         * 2023) processors can do a lot in 10ms and that it's just below what