On Sat, Sep 8, 2012 at 7:41 AM, Tejun Heo <t...@kernel.org> wrote:
> I think this should do it. Can you spot any hole with the following
> patch?
>
> Thanks.
>
> Index: work/kernel/workqueue.c
> ===================================================================
> --- work.orig/kernel/workqueue.c
> +++ work/kernel/workqueue.c
> @@ -66,6 +66,7 @@ enum {
>
> /* pool flags */
> POOL_MANAGE_WORKERS = 1 << 0, /* need to manage workers */
> + POOL_MANAGING_WORKERS = 1 << 1, /* managing workers */
>
> /* worker flags */
> WORKER_STARTED = 1 << 0, /* started */
> @@ -165,7 +166,7 @@ struct worker_pool {
> struct timer_list idle_timer; /* L: worker idle timeout */
> struct timer_list mayday_timer; /* L: SOS timer for workers */
>
> - struct mutex manager_mutex; /* mutex manager should hold
> */
> + struct mutex manager_mutex; /* manager <-> CPU hotplug */
> struct ida worker_ida; /* L: for worker IDs */
> };
>
> @@ -480,6 +481,7 @@ static atomic_t unbound_pool_nr_running[
> };
>
> static int worker_thread(void *__worker);
> +static void process_scheduled_works(struct worker *worker);
>
> static int worker_pool_pri(struct worker_pool *pool)
> {
> @@ -652,7 +654,7 @@ static bool need_to_manage_workers(struc
> /* Do we have too many workers and should some go away? */
> static bool too_many_workers(struct worker_pool *pool)
> {
> - bool managing = mutex_is_locked(&pool->manager_mutex);
> + bool managing = pool->flags & POOL_MANAGING_WORKERS;
> int nr_idle = pool->nr_idle + managing; /* manager is considered idle
> */
> int nr_busy = pool->nr_workers - nr_idle;
>
> @@ -1820,14 +1822,43 @@ static bool maybe_destroy_workers(struct
> * some action was taken.
> */
> static bool manage_workers(struct worker *worker)
> + __releases(&gcwq->lock) __acquires(&gcwq->lock)
> {
> struct worker_pool *pool = worker->pool;
> + struct global_cwq *gcwq = pool->gcwq;
> bool ret = false;
>
> - if (!mutex_trylock(&pool->manager_mutex))
> - return ret;
> + if (pool->flags & POOL_MANAGING_WORKERS)
> + return ret;
>
> pool->flags &= ~POOL_MANAGE_WORKERS;
> + pool->flags |= POOL_MANAGING_WORKERS;
> +
> + /*
> + * To simplify both worker management and CPU hotplug, hold off
> + * management while hotplug is in progress. CPU hotplug path can't
> + * grab %POOL_MANAGING_WORKERS 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.
> + *
> + * manager_mutex would always be free unless CPU hotplug is in
> + * progress. trylock first without dropping gcwq->lock.
> + */
> + if (unlikely(!mutex_trylock(&pool->manager_mutex))) {
> + spin_unlock_irq(&gcwq->lock);
hotplug can happen here.
> + mutex_lock(&pool->manager_mutex);
> + spin_lock_irq(&gcwq->lock);
> +
> + /*
> + * CPU hotplug could have scheduled rebind_work while we're
> + * waiting for manager_mutex. Rebind before doing anything
> + * else. This has to be handled here. worker_thread()
> + * will be confused by the unexpected work item.
> + */
> + process_scheduled_works(worker);
hotplug code can't iterate manager. not rebind_work() nor UNBOUND for manager.
> + ret = true;
> + }
>
> /*
> * Destroy and then create so that may_start_working() is true
> @@ -1836,7 +1867,9 @@ static bool manage_workers(struct worker
> ret |= maybe_destroy_workers(pool);
> ret |= maybe_create_worker(pool);
>
> + pool->flags &= ~POOL_MANAGING_WORKERS;
> mutex_unlock(&pool->manager_mutex);
> +
> return ret;
> }
>
> @@ -3393,7 +3426,7 @@ EXPORT_SYMBOL_GPL(work_busy);
> * cpu comes back online.
> */
>
> -/* claim manager positions of all pools */
> +/* claim manager positions of all pools, see manage_workers() for details */
> static void gcwq_claim_management_and_lock(struct global_cwq *gcwq)
> {
> struct worker_pool *pool;
> --
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