workqueue: use mutex for global_cwq manager exclusion

POOL_MANAGING_WORKERS is used to ensure that at most one worker takes
the manager role at any given time on a given global_cwq.  Trustee
later hitched on it to assume manager adding blocking wait for the
bit.  As trustee already needed a custom wait mechanism, waiting for
MANAGING_WORKERS was rolled into the same mechanism.

Trustee is scheduled to be removed.  This patch separates out
MANAGING_WORKERS wait into per-pool mutex.  Workers use
mutex_trylock() to test for manager role and trustee uses mutex_lock()
to claim manager roles.

gcwq_claim/release_management() helpers are added to grab and release
manager roles of all pools on a global_cwq.  gcwq_claim_management()
always grabs pool manager mutexes in ascending pool index order and
uses pool index as lockdep subclass.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>

1 files changed, 26 insertions, 39 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index af512927c607..f7a00697d150 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -51,7 +51,6 @@ 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 */
@@ -155,6 +154,7 @@ 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 ida              worker_ida;     /* L: for worker IDs */
         struct worker           *first_idle;    /* L: first idle worker */
 };
@@ -644,7 +644,7 @@ static bool need_to_manage_workers(struct worker_pool *pool)
 /* Do we have too many workers and should some go away? */
 static bool too_many_workers(struct worker_pool *pool)
 {
-        bool managing = pool->flags & POOL_MANAGING_WORKERS;
+        bool managing = mutex_is_locked(&pool->manager_mutex);
         int nr_idle = pool->nr_idle + managing; /* manager is considered idle */
         int nr_busy = pool->nr_workers - nr_idle;
 
@@ -1655,14 +1655,12 @@ static bool maybe_destroy_workers(struct worker_pool *pool)
 static bool manage_workers(struct worker *worker)
 {
         struct worker_pool *pool = worker->pool;
-        struct global_cwq *gcwq = pool->gcwq;
         bool ret = false;
 
-        if (pool->flags & POOL_MANAGING_WORKERS)
+        if (!mutex_trylock(&pool->manager_mutex))
                 return ret;
 
         pool->flags &= ~POOL_MANAGE_WORKERS;
-        pool->flags |= POOL_MANAGING_WORKERS;
 
         /*
          * Destroy and then create so that may_start_working() is true
@@ -1671,15 +1669,7 @@ static bool manage_workers(struct worker *worker)
         ret |= maybe_destroy_workers(pool);
         ret |= maybe_create_worker(pool);
 
-        pool->flags &= ~POOL_MANAGING_WORKERS;
-
-        /*
-         * The trustee might be waiting to take over the manager
-         * position, tell it we're done.
-         */
-        if (unlikely(gcwq->trustee))
-                wake_up_all(&gcwq->trustee_wait);
-
+        mutex_unlock(&pool->manager_mutex);
         return ret;
 }
 
@@ -3255,6 +3245,24 @@ EXPORT_SYMBOL_GPL(work_busy);
  *                         ----------------> RELEASE --------------
  */
 
+/* claim manager positions of all pools */
+static void gcwq_claim_management(struct global_cwq *gcwq)
+{
+        struct worker_pool *pool;
+
+        for_each_worker_pool(pool, gcwq)
+                mutex_lock_nested(&pool->manager_mutex, pool - gcwq->pools);
+}
+
+/* release manager positions */
+static void gcwq_release_management(struct global_cwq *gcwq)
+{
+        struct worker_pool *pool;
+
+        for_each_worker_pool(pool, gcwq)
+                mutex_unlock(&pool->manager_mutex);
+}
+
 /**
  * trustee_wait_event_timeout - timed event wait for trustee
  * @cond: condition to wait for
@@ -3304,16 +3312,6 @@ EXPORT_SYMBOL_GPL(work_busy);
         __ret1 < 0 ? -1 : 0;                                            \
 })
 
-static bool gcwq_is_managing_workers(struct global_cwq *gcwq)
-{
-        struct worker_pool *pool;
-
-        for_each_worker_pool(pool, gcwq)
-                if (pool->flags & POOL_MANAGING_WORKERS)
-                        return true;
-        return false;
-}
-
 static bool gcwq_has_idle_workers(struct global_cwq *gcwq)
 {
         struct worker_pool *pool;
@@ -3336,15 +3334,8 @@ static int __cpuinit trustee_thread(void *__gcwq)
 
         BUG_ON(gcwq->cpu != smp_processor_id());
 
+        gcwq_claim_management(gcwq);
         spin_lock_irq(&gcwq->lock);
-        /*
-         * Claim the manager position and make all workers rogue.
-         * Trustee must be bound to the target cpu and can't be
-         * cancelled.
-         */
-        BUG_ON(gcwq->cpu != smp_processor_id());
-        rc = trustee_wait_event(!gcwq_is_managing_workers(gcwq));
-        BUG_ON(rc < 0);
 
         /*
          * We've claimed all manager positions.  Make all workers unbound
@@ -3352,12 +3343,9 @@ static int __cpuinit trustee_thread(void *__gcwq)
          * ones which are still executing works from before the last CPU
          * down must be on the cpu.  After this, they may become diasporas.
          */
-        for_each_worker_pool(pool, gcwq) {
-                pool->flags |= POOL_MANAGING_WORKERS;
-
+        for_each_worker_pool(pool, gcwq)
                 list_for_each_entry(worker, &pool->idle_list, entry)
                         worker->flags |= WORKER_UNBOUND;
-        }
 
         for_each_busy_worker(worker, i, pos, gcwq)
                 worker->flags |= WORKER_UNBOUND;
@@ -3497,9 +3485,7 @@ static int __cpuinit trustee_thread(void *__gcwq)
                             work_color_to_flags(WORK_NO_COLOR));
         }
 
-        /* relinquish manager role */
-        for_each_worker_pool(pool, gcwq)
-                pool->flags &= ~POOL_MANAGING_WORKERS;
+        gcwq_release_management(gcwq);
 
         /* notify completion */
         gcwq->trustee = NULL;
@@ -3894,6 +3880,7 @@ static int __init init_workqueues(void)
                         setup_timer(&pool->mayday_timer, gcwq_mayday_timeout,
                                     (unsigned long)pool);
 
+                        mutex_init(&pool->manager_mutex);
                         ida_init(&pool->worker_ida);
                 }