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Date: Fri, 15 Mar 2013 00:00:44 +0800
From: Lai Jiangshan <eag0628@...il.com>
To: Tejun Heo <tj@...nel.org>
Cc: linux-kernel@...r.kernel.org, laijs@...fujitsu.com
Subject: Re: [PATCH 1/7] workqueue: rename worker_pool->assoc_mutex to ->manager_mutex
On Thu, Mar 14, 2013 at 10:57 AM, Tejun Heo <tj@...nel.org> wrote:
> Manager operations are currently governed by two mutexes -
> pool->manager_arb and ->assoc_mutex. The former is used to decide who
> gets to be the manager and the latter to exclude the actual manager
> operations including creation and destruction of workers. Anyone who
> grabs ->manager_arb must perform manager role; otherwise, the pool
> might stall.
>
> Grabbing ->assoc_mutex blocks everyone else from performing manager
> operations but doesn't require the holder to perform manager duties as
> it's merely blocking manager operations without becoming the manager.
>
> Because the blocking was necessary when [dis]associating per-cpu
> workqueues during CPU hotplug events, the latter was named
> assoc_mutex. The mutex is scheduled to be used for other purposes, so
> this patch gives it a more fitting generic name - manager_mutex - and
> updates / adds comments to explain synchronization around the manager
> role and operations.
>
> This patch is pure rename / doc update.
>
> Signed-off-by: Tejun Heo <tj@...nel.org>
> ---
> kernel/workqueue.c | 62 +++++++++++++++++++++++++++++++++---------------------
> 1 file changed, 38 insertions(+), 24 deletions(-)
>
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index f37421f..bc25bdf 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);
Hi, Tejun
It must conflicts with wq/for-3.9-fixes.
Thanks,
Lai
> }
>
> /*
> @@ -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;
> }
> --
> 1.8.1.4
>
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