diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index f37421fb4f35..bc25bdfb4b42 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -60,8 +60,8 @@ enum {
* %WORKER_UNBOUND set and concurrency management disabled, and may
* be executing on any CPU. The pool behaves as an unbound one.
*
- * Note that DISASSOCIATED can be flipped only while holding
- * assoc_mutex to avoid changing binding state while
+ * Note that DISASSOCIATED should be flipped only while holding
+ * manager_mutex to avoid changing binding state while
* create_worker() is in progress.
*/
POOL_MANAGE_WORKERS = 1 << 0, /* need to manage workers */
@@ -149,8 +149,9 @@ struct worker_pool {
DECLARE_HASHTABLE(busy_hash, BUSY_WORKER_HASH_ORDER);
/* L: hash of busy workers */
+ /* see manage_workers() for details on the two manager mutexes */
struct mutex manager_arb; /* manager arbitration */
- struct mutex assoc_mutex; /* protect POOL_DISASSOCIATED */
+ struct mutex manager_mutex; /* manager exclusion */
struct ida worker_ida; /* L: for worker IDs */
struct workqueue_attrs *attrs; /* I: worker attributes */
@@ -1635,7 +1636,7 @@ static void rebind_workers(struct worker_pool *pool)
struct worker *worker, *n;
int i;
- lockdep_assert_held(&pool->assoc_mutex);
+ lockdep_assert_held(&pool->manager_mutex);
lockdep_assert_held(&pool->lock);
/* dequeue and kick idle ones */
@@ -2022,31 +2023,44 @@ static bool manage_workers(struct worker *worker)
struct worker_pool *pool = worker->pool;
bool ret = false;
+ /*
+ * Managership is governed by two mutexes - manager_arb and
+ * manager_mutex. manager_arb handles arbitration of manager role.
+ * Anyone who successfully grabs manager_arb wins the arbitration
+ * and becomes the manager. mutex_trylock() on pool->manager_arb
+ * failure while holding pool->lock reliably indicates that someone
+ * else is managing the pool and the worker which failed trylock
+ * can proceed to executing work items. This means that anyone
+ * grabbing manager_arb is responsible for actually performing
+ * manager duties. If manager_arb is grabbed and released without
+ * actual management, the pool may stall indefinitely.
+ *
+ * manager_mutex is used for exclusion of actual management
+ * operations. The holder of manager_mutex can be sure that none
+ * of management operations, including creation and destruction of
+ * workers, won't take place until the mutex is released. Because
+ * manager_mutex doesn't interfere with manager role arbitration,
+ * it is guaranteed that the pool's management, while may be
+ * delayed, won't be disturbed by someone else grabbing
+ * manager_mutex.
+ */
if (!mutex_trylock(&pool->manager_arb))
return ret;
/*
- * To simplify both worker management and CPU hotplug, hold off
- * management while hotplug is in progress. CPU hotplug path can't
- * grab @pool->manager_arb to achieve this because that can lead to
- * idle worker depletion (all become busy thinking someone else is
- * managing) which in turn can result in deadlock under extreme
- * circumstances. Use @pool->assoc_mutex to synchronize manager
- * against CPU hotplug.
- *
- * assoc_mutex would always be free unless CPU hotplug is in
- * progress. trylock first without dropping @pool->lock.
+ * With manager arbitration won, manager_mutex would be free in
+ * most cases. trylock first without dropping @pool->lock.
*/
- if (unlikely(!mutex_trylock(&pool->assoc_mutex))) {
+ if (unlikely(!mutex_trylock(&pool->manager_mutex))) {
spin_unlock_irq(&pool->lock);
- mutex_lock(&pool->assoc_mutex);
+ mutex_lock(&pool->manager_mutex);
/*
* CPU hotplug could have happened while we were waiting
* for assoc_mutex. Hotplug itself can't handle us
* because manager isn't either on idle or busy list, and
* @pool's state and ours could have deviated.
*
- * As hotplug is now excluded via assoc_mutex, we can
+ * As hotplug is now excluded via manager_mutex, we can
* simply try to bind. It will succeed or fail depending
* on @pool's current state. Try it and adjust
* %WORKER_UNBOUND accordingly.
@@ -2068,7 +2082,7 @@ static bool manage_workers(struct worker *worker)
ret |= maybe_destroy_workers(pool);
ret |= maybe_create_worker(pool);
- mutex_unlock(&pool->assoc_mutex);
+ mutex_unlock(&pool->manager_mutex);
mutex_unlock(&pool->manager_arb);
return ret;
}
@@ -3436,7 +3450,7 @@ static int init_worker_pool(struct worker_pool *pool)
(unsigned long)pool);
mutex_init(&pool->manager_arb);
- mutex_init(&pool->assoc_mutex);
+ mutex_init(&pool->manager_mutex);
ida_init(&pool->worker_ida);
INIT_HLIST_NODE(&pool->hash_node);
@@ -4076,11 +4090,11 @@ static void wq_unbind_fn(struct work_struct *work)
for_each_cpu_worker_pool(pool, cpu) {
WARN_ON_ONCE(cpu != smp_processor_id());
- mutex_lock(&pool->assoc_mutex);
+ mutex_lock(&pool->manager_mutex);
spin_lock_irq(&pool->lock);
/*
- * We've claimed all manager positions. Make all workers
+ * We've blocked all manager operations. Make all workers
* unbound and set DISASSOCIATED. Before this, all workers
* except for the ones which are still executing works from
* before the last CPU down must be on the cpu. After
@@ -4095,7 +4109,7 @@ static void wq_unbind_fn(struct work_struct *work)
pool->flags |= POOL_DISASSOCIATED;
spin_unlock_irq(&pool->lock);
- mutex_unlock(&pool->assoc_mutex);
+ mutex_unlock(&pool->manager_mutex);
}
/*
@@ -4152,14 +4166,14 @@ static int __cpuinit workqueue_cpu_up_callback(struct notifier_block *nfb,
case CPU_DOWN_FAILED:
case CPU_ONLINE:
for_each_cpu_worker_pool(pool, cpu) {
- mutex_lock(&pool->assoc_mutex);
+ mutex_lock(&pool->manager_mutex);
spin_lock_irq(&pool->lock);
pool->flags &= ~POOL_DISASSOCIATED;
rebind_workers(pool);
spin_unlock_irq(&pool->lock);
- mutex_unlock(&pool->assoc_mutex);
+ mutex_unlock(&pool->manager_mutex);
}
break;
}