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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>
This commit is contained in:
Tejun Heo 2023-08-07 15:57:23 -10:00
parent fcecfa8f27
commit 967b494e2f
1 changed files with 23 additions and 17 deletions
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40
kernel/workqueue.c
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@ -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