Merge branch 'for-3.6-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq into for-3.7

This merge is necessary as Lai's CPU hotplug restructuring series
depends on the CPU hotplug bug fixes in for-3.6-fixes.

The merge creates one trivial conflict between the following two
commits.

 96e65306b8 "workqueue: UNBOUND -> REBIND morphing in rebind_workers() should be atomic"
 e2b6a6d570 "workqueue: use system_highpri_wq for highpri workers in rebind_workers()"

Both add local variable definitions to the same block and can be
merged in any order.

Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 99 insertions, 23 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 039d0fae171a..31d8a4586d4c 100644
--- a/kernel/workqueue.c
+++ b/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 */
@@ -682,7 +683,7 @@ 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 = 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;
 
@@ -1632,6 +1633,15 @@ static void idle_worker_rebind(struct worker *worker)
 
         /* we did our part, wait for rebind_workers() to finish up */
         wait_event(gcwq->rebind_hold, !(worker->flags & WORKER_REBIND));
+
+        /*
+         * rebind_workers() shouldn't finish until all workers passed the
+         * above WORKER_REBIND wait.  Tell it when done.
+         */
+        spin_lock_irq(&worker->pool->gcwq->lock);
+        if (!--worker->idle_rebind->cnt)
+                complete(&worker->idle_rebind->done);
+        spin_unlock_irq(&worker->pool->gcwq->lock);
 }
 
 /*
@@ -1645,8 +1655,16 @@ static void busy_worker_rebind_fn(struct work_struct *work)
         struct worker *worker = container_of(work, struct worker, rebind_work);
         struct global_cwq *gcwq = worker->pool->gcwq;
 
-        if (worker_maybe_bind_and_lock(worker))
-                worker_clr_flags(worker, WORKER_REBIND);
+        worker_maybe_bind_and_lock(worker);
+
+        /*
+         * %WORKER_REBIND must be cleared even if the above binding failed;
+         * otherwise, we may confuse the next CPU_UP cycle or oops / get
+         * stuck by calling idle_worker_rebind() prematurely.  If CPU went
+         * down again inbetween, %WORKER_UNBOUND would be set, so clearing
+         * %WORKER_REBIND is always safe.
+         */
+        worker_clr_flags(worker, WORKER_REBIND);
 
         spin_unlock_irq(&gcwq->lock);
 }
@@ -1702,12 +1720,15 @@ retry:
         /* set REBIND and kick idle ones, we'll wait for these later */
         for_each_worker_pool(pool, gcwq) {
                 list_for_each_entry(worker, &pool->idle_list, entry) {
+                        unsigned long worker_flags = worker->flags;
+
                         if (worker->flags & WORKER_REBIND)
                                 continue;
 
-                        /* morph UNBOUND to REBIND */
-                        worker->flags &= ~WORKER_UNBOUND;
-                        worker->flags |= WORKER_REBIND;
+                        /* morph UNBOUND to REBIND atomically */
+                        worker_flags &= ~WORKER_UNBOUND;
+                        worker_flags |= WORKER_REBIND;
+                        ACCESS_ONCE(worker->flags) = worker_flags;
 
                         idle_rebind.cnt++;
                         worker->idle_rebind = &idle_rebind;
@@ -1725,26 +1746,16 @@ retry:
                 goto retry;
         }
 
-        /*
-         * All idle workers are rebound and waiting for %WORKER_REBIND to
-         * be cleared inside idle_worker_rebind().  Clear and release.
-         * Clearing %WORKER_REBIND from this foreign context is safe
-         * because these workers are still guaranteed to be idle.
-         */
-        for_each_worker_pool(pool, gcwq)
-                list_for_each_entry(worker, &pool->idle_list, entry)
-                        worker->flags &= ~WORKER_REBIND;
-
-        wake_up_all(&gcwq->rebind_hold);
-
-        /* rebind busy workers */
+        /* all idle workers are rebound, rebind busy workers */
         for_each_busy_worker(worker, i, pos, gcwq) {
+                unsigned long worker_flags = worker->flags;
                 struct work_struct *rebind_work = &worker->rebind_work;
                 struct workqueue_struct *wq;
 
-                /* morph UNBOUND to REBIND */
-                worker->flags &= ~WORKER_UNBOUND;
-                worker->flags |= WORKER_REBIND;
+                /* morph UNBOUND to REBIND atomically */
+                worker_flags &= ~WORKER_UNBOUND;
+                worker_flags |= WORKER_REBIND;
+                ACCESS_ONCE(worker->flags) = worker_flags;
 
                 if (test_and_set_bit(WORK_STRUCT_PENDING_BIT,
                                      work_data_bits(rebind_work)))
@@ -1765,6 +1776,34 @@ retry:
                         worker->scheduled.next,
                         work_color_to_flags(WORK_NO_COLOR));
         }
+
+        /*
+         * All idle workers are rebound and waiting for %WORKER_REBIND to
+         * be cleared inside idle_worker_rebind().  Clear and release.
+         * Clearing %WORKER_REBIND from this foreign context is safe
+         * because these workers are still guaranteed to be idle.
+         *
+         * We need to make sure all idle workers passed WORKER_REBIND wait
+         * in idle_worker_rebind() before returning; otherwise, workers can
+         * get stuck at the wait if hotplug cycle repeats.
+         */
+        idle_rebind.cnt = 1;
+        INIT_COMPLETION(idle_rebind.done);
+
+        for_each_worker_pool(pool, gcwq) {
+                list_for_each_entry(worker, &pool->idle_list, entry) {
+                        worker->flags &= ~WORKER_REBIND;
+                        idle_rebind.cnt++;
+                }
+        }
+
+        wake_up_all(&gcwq->rebind_hold);
+
+        if (--idle_rebind.cnt) {
+                spin_unlock_irq(&gcwq->lock);
+                wait_for_completion(&idle_rebind.done);
+                spin_lock_irq(&gcwq->lock);
+        }
 }
 
 static struct worker *alloc_worker(void)
@@ -2110,9 +2149,45 @@ static bool manage_workers(struct worker *worker)
         struct worker_pool *pool = worker->pool;
         bool ret = false;
 
-        if (!mutex_trylock(&pool->manager_mutex))
+        if (pool->flags & POOL_MANAGING_WORKERS)
                 return ret;
 
+        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.  Use @pool->manager_mutex to synchronize
+         * manager against CPU hotplug.
+         *
+         * 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(&pool->gcwq->lock);
+                mutex_lock(&pool->manager_mutex);
+                /*
+                 * CPU hotplug could have happened while we were waiting
+                 * for manager_mutex.  Hotplug itself can't handle us
+                 * because manager isn't either on idle or busy list, and
+                 * @gcwq's state and ours could have deviated.
+                 *
+                 * As hotplug is now excluded via manager_mutex, we can
+                 * simply try to bind.  It will succeed or fail depending
+                 * on @gcwq's current state.  Try it and adjust
+                 * %WORKER_UNBOUND accordingly.
+                 */
+                if (worker_maybe_bind_and_lock(worker))
+                        worker->flags &= ~WORKER_UNBOUND;
+                else
+                        worker->flags |= WORKER_UNBOUND;
+
+                ret = true;
+        }
+
         pool->flags &= ~POOL_MANAGE_WORKERS;
 
         /*
@@ -2122,6 +2197,7 @@ static bool manage_workers(struct worker *worker)
         ret |= maybe_destroy_workers(pool);
         ret |= maybe_create_worker(pool);
 
+        pool->flags &= ~POOL_MANAGING_WORKERS;
         mutex_unlock(&pool->manager_mutex);
         return ret;
 }