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Subject[PATCH 23/31] workqueue: implement get/put_pwq()
Date
Add pool_workqueue->refcnt along with get/put_pwq().  Both per-cpu and
unbound pwqs have refcnts and any work item inserted on a pwq
increments the refcnt which is dropped when the work item finishes.

For per-cpu pwqs the base ref is never dropped and destroy_workqueue()
frees the pwqs as before. For unbound ones, destroy_workqueue()
simply drops the base ref on the first pwq. When the refcnt reaches
zero, pwq_unbound_release_workfn() is scheduled on system_wq, which
unlinks the pwq, puts the associated pool and frees the pwq and wq as
necessary. This needs to be done from a work item as put_pwq() needs
to be protected by pool->lock but release can't happen with the lock
held - e.g. put_unbound_pool() involves blocking operations.

Unbound pool->locks are marked with lockdep subclas 1 as put_pwq()
will schedule the release work item on system_wq while holding the
unbound pool's lock and triggers recursive locking warning spuriously.

This will be used to implement dynamic creation and destruction of
unbound pwqs.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
kernel/workqueue.c | 137 ++++++++++++++++++++++++++++++++++++++++++++---------
1 file changed, 114 insertions(+), 23 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index d0604ee..e092cd5 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -179,6 +179,7 @@ struct pool_workqueue {
struct workqueue_struct *wq; /* I: the owning workqueue */
int work_color; /* L: current color */
int flush_color; /* L: flushing color */
+ int refcnt; /* L: reference count */
int nr_in_flight[WORK_NR_COLORS];
/* L: nr of in_flight works */
int nr_active; /* L: nr of active works */
@@ -186,6 +187,15 @@ struct pool_workqueue {
struct list_head delayed_works; /* L: delayed works */
struct list_head pwqs_node; /* R: node on wq->pwqs */
struct list_head mayday_node; /* W: node on wq->maydays */
+
+ /*
+ * Release of unbound pwq is punted to system_wq. See put_pwq()
+ * and pwq_unbound_release_workfn() for details. pool_workqueue
+ * itself is also sched-RCU protected so that the first pwq can be
+ * determined without grabbing workqueue_lock.
+ */
+ struct work_struct unbound_release_work;
+ struct rcu_head rcu;
} __aligned(1 << WORK_STRUCT_FLAG_BITS);

/*
@@ -936,6 +946,45 @@ static void move_linked_works(struct work_struct *work, struct list_head *head,
*nextp = n;
}

+/**
+ * get_pwq - get an extra reference on the specified pool_workqueue
+ * @pwq: pool_workqueue to get
+ *
+ * Obtain an extra reference on @pwq. The caller should guarantee that
+ * @pwq has positive refcnt and be holding the matching pool->lock.
+ */
+static void get_pwq(struct pool_workqueue *pwq)
+{
+ lockdep_assert_held(&pwq->pool->lock);
+ WARN_ON_ONCE(pwq->refcnt <= 0);
+ pwq->refcnt++;
+}
+
+/**
+ * put_pwq - put a pool_workqueue reference
+ * @pwq: pool_workqueue to put
+ *
+ * Drop a reference of @pwq. If its refcnt reaches zero, schedule its
+ * destruction. The caller should be holding the matching pool->lock.
+ */
+static void put_pwq(struct pool_workqueue *pwq)
+{
+ lockdep_assert_held(&pwq->pool->lock);
+ if (likely(--pwq->refcnt))
+ return;
+ 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().
+ */
+ schedule_work(&pwq->unbound_release_work);
+}
+
static void pwq_activate_delayed_work(struct work_struct *work)
{
struct pool_workqueue *pwq = get_work_pwq(work);
@@ -967,9 +1016,9 @@ static void pwq_activate_first_delayed(struct pool_workqueue *pwq)
*/
static void pwq_dec_nr_in_flight(struct pool_workqueue *pwq, int color)
{
- /* ignore uncolored works */
+ /* uncolored work items don't participate in flushing or nr_active */
if (color == WORK_NO_COLOR)
- return;
+ goto out_put;

pwq->nr_in_flight[color]--;

@@ -982,11 +1031,11 @@ static void pwq_dec_nr_in_flight(struct pool_workqueue *pwq, int color)

/* is flush in progress and are we at the flushing tip? */
if (likely(pwq->flush_color != color))
- return;
+ goto out_put;

/* are there still in-flight works? */
if (pwq->nr_in_flight[color])
- return;
+ goto out_put;

/* this pwq is done, clear flush_color */
pwq->flush_color = -1;
@@ -997,6 +1046,8 @@ static void pwq_dec_nr_in_flight(struct pool_workqueue *pwq, int color)
*/
if (atomic_dec_and_test(&pwq->wq->nr_pwqs_to_flush))
complete(&pwq->wq->first_flusher->done);
+out_put:
+ put_pwq(pwq);
}

/**
@@ -1119,6 +1170,7 @@ static void insert_work(struct pool_workqueue *pwq, struct work_struct *work,
/* we own @work, set data and link */
set_work_pwq(work, pwq, extra_flags);
list_add_tail(&work->entry, head);
+ get_pwq(pwq);

/*
* Ensure either worker_sched_deactivated() sees the above
@@ -3294,6 +3346,7 @@ 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);

if (worker_pool_assign_id(pool) < 0)
@@ -3322,7 +3375,41 @@ fail:
return NULL;
}

-/* initialize @pwq which interfaces with @pool for @wq and link it in */
+static void rcu_free_pwq(struct rcu_head *rcu)
+{
+ kmem_cache_free(pwq_cache,
+ container_of(rcu, struct pool_workqueue, rcu));
+}
+
+/*
+ * Scheduled on system_wq 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)
+{
+ struct pool_workqueue *pwq = container_of(work, struct pool_workqueue,
+ unbound_release_work);
+ struct workqueue_struct *wq = pwq->wq;
+ struct worker_pool *pool = pwq->pool;
+
+ if (WARN_ON_ONCE(!(wq->flags & WQ_UNBOUND)))
+ return;
+
+ spin_lock_irq(&workqueue_lock);
+ list_del_rcu(&pwq->pwqs_node);
+ spin_unlock_irq(&workqueue_lock);
+
+ put_unbound_pool(pool);
+ call_rcu_sched(&pwq->rcu, rcu_free_pwq);
+
+ /*
+ * If we're the last pwq going away, @wq is already dead and no one
+ * is gonna access it anymore. Free it.
+ */
+ if (list_empty(&wq->pwqs))
+ kfree(wq);
+}
+
static void init_and_link_pwq(struct pool_workqueue *pwq,
struct workqueue_struct *wq,
struct worker_pool *pool)
@@ -3332,9 +3419,11 @@ static void init_and_link_pwq(struct pool_workqueue *pwq,
pwq->pool = pool;
pwq->wq = wq;
pwq->flush_color = -1;
+ pwq->refcnt = 1;
pwq->max_active = wq->saved_max_active;
INIT_LIST_HEAD(&pwq->delayed_works);
INIT_LIST_HEAD(&pwq->mayday_node);
+ INIT_WORK(&pwq->unbound_release_work, pwq_unbound_release_workfn);

list_add_tail_rcu(&pwq->pwqs_node, &wq->pwqs);
}
@@ -3377,15 +3466,6 @@ static int alloc_and_link_pwqs(struct workqueue_struct *wq)
return 0;
}

-static void free_pwqs(struct workqueue_struct *wq)
-{
- if (!(wq->flags & WQ_UNBOUND))
- free_percpu(wq->cpu_pwqs);
- else if (!list_empty(&wq->pwqs))
- kmem_cache_free(pwq_cache, list_first_entry(&wq->pwqs,
- struct pool_workqueue, pwqs_node));
-}
-
static int wq_clamp_max_active(int max_active, unsigned int flags,
const char *name)
{
@@ -3517,7 +3597,8 @@ void destroy_workqueue(struct workqueue_struct *wq)
}
}

- if (WARN_ON(pwq->nr_active) ||
+ if (WARN_ON(pwq->refcnt > 1) ||
+ WARN_ON(pwq->nr_active) ||
WARN_ON(!list_empty(&pwq->delayed_works))) {
spin_unlock_irq(&workqueue_lock);
return;
@@ -3538,17 +3619,27 @@ void destroy_workqueue(struct workqueue_struct *wq)
wq->rescuer = NULL;
}

- /*
- * We're the sole accessor of @wq at this point. Directly access
- * the first pwq and put its pool.
- */
- if (wq->flags & WQ_UNBOUND) {
+ if (!(wq->flags & WQ_UNBOUND)) {
+ /*
+ * The base ref is never dropped on per-cpu pwqs. Directly
+ * free the pwqs and wq.
+ */
+ free_percpu(wq->cpu_pwqs);
+ kfree(wq);
+ } else {
+ /*
+ * We're the sole accessor of @wq at this point. Directly
+ * access the first pwq and put the base ref. As both pwqs
+ * and pools are sched-RCU protected, the lock operations
+ * are safe. @wq will be freed when the last pwq is
+ * released.
+ */
pwq = list_first_entry(&wq->pwqs, struct pool_workqueue,
pwqs_node);
- put_unbound_pool(pwq->pool);
+ spin_lock_irq(&pwq->pool->lock);
+ put_pwq(pwq);
+ spin_unlock_irq(&pwq->pool->lock);
}
- free_pwqs(wq);
- kfree(wq);
}
EXPORT_SYMBOL_GPL(destroy_workqueue);

--
1.8.1.2


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