workqueue: Prevent deadlock/stall on RT

Austin reported a XFS deadlock/stall on RT where scheduled work gets
never exececuted and tasks are waiting for each other for ever.

The underlying problem is the modification of the RT code to the
handling of workers which are about to go to sleep. In mainline a
worker thread which goes to sleep wakes an idle worker if there is
more work to do. This happens from the guts of the schedule()
function. On RT this must be outside and the accessed data structures
are not protected against scheduling due to the spinlock to rtmutex
conversion. So the naive solution to this was to move the code outside
of the scheduler and protect the data structures by the pool
lock. That approach turned out to be a little naive as we cannot call
into that code when the thread blocks on a lock, as it is not allowed
to block on two locks in parallel. So we dont call into the worker
wakeup magic when the worker is blocked on a lock, which causes the
deadlock/stall observed by Austin and Mike.

Looking deeper into that worker code it turns out that the only
relevant data structure which needs to be protected is the list of
idle workers which can be woken up.

So the solution is to protect the list manipulation operations with
preempt_enable/disable pairs on RT and call unconditionally into the
worker code even when the worker is blocked on a lock. The preemption
protection is safe as there is nothing which can fiddle with the list
outside of thread context.

Reported-and_tested-by: Austin Schuh <austin@peloton-tech.com>
Reported-and_tested-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: http://vger.kernel.org/r/alpine.DEB.2.10.1406271249510.5170@nanos
Cc: Richard Weinberger <richard.weinberger@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: stable-rt@vger.kernel.org

2 files changed, 53 insertions, 15 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 52d1966d2838..33b015814dfc 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2911,9 +2911,8 @@ need_resched:
 
 static inline void sched_submit_work(struct task_struct *tsk)
 {
-	if (!tsk->state || tsk_is_pi_blocked(tsk))
+	if (!tsk->state)
 		return;
-
 	/*
 	 * If a worker went to sleep, notify and ask workqueue whether
 	 * it wants to wake up a task to maintain concurrency.
@@ -2921,6 +2920,10 @@ static inline void sched_submit_work(struct task_struct *tsk)
 	if (tsk->flags & PF_WQ_WORKER)
 		wq_worker_sleeping(tsk);
 
+
+	if (tsk_is_pi_blocked(tsk))
+		return;
+
 	/*
 	 * If we are going to sleep and we have plugged IO queued,
 	 * make sure to submit it to avoid deadlocks.
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 8d83b38727b5..54165e1a5cb5 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -126,6 +126,11 @@ enum {
  *    cpu or grabbing pool->lock is enough for read access.  If
  *    POOL_DISASSOCIATED is set, it's identical to L.
  *
+ *    On RT we need the extra protection via rt_lock_idle_list() for
+ *    the list manipulations against read access from
+ *    wq_worker_sleeping(). All other places are nicely serialized via
+ *    pool->lock.
+ *
  * MG: pool->manager_mutex and pool->lock protected.  Writes require both
  *     locks.  Reads can happen under either lock.
  *
@@ -409,6 +414,31 @@ static void copy_workqueue_attrs(struct workqueue_attrs *to,
 		if (({ assert_rcu_or_wq_mutex(wq); false; })) { }	\
 		else
 
+#ifdef CONFIG_PREEMPT_RT_BASE
+static inline void rt_lock_idle_list(struct worker_pool *pool)
+{
+	preempt_disable();
+}
+static inline void rt_unlock_idle_list(struct worker_pool *pool)
+{
+	preempt_enable();
+}
+static inline void sched_lock_idle_list(struct worker_pool *pool) { }
+static inline void sched_unlock_idle_list(struct worker_pool *pool) { }
+#else
+static inline void rt_lock_idle_list(struct worker_pool *pool) { }
+static inline void rt_unlock_idle_list(struct worker_pool *pool) { }
+static inline void sched_lock_idle_list(struct worker_pool *pool)
+{
+	spin_lock_irq(&pool->lock);
+}
+static inline void sched_unlock_idle_list(struct worker_pool *pool)
+{
+	spin_unlock_irq(&pool->lock);
+}
+#endif
+
+
 #ifdef CONFIG_DEBUG_OBJECTS_WORK
 
 static struct debug_obj_descr work_debug_descr;
@@ -808,10 +838,16 @@ static struct worker *first_worker(struct worker_pool *pool)
  */
 static void wake_up_worker(struct worker_pool *pool)
 {
-	struct worker *worker = first_worker(pool);
+	struct worker *worker;
+
+	rt_lock_idle_list(pool);
+
+	worker = first_worker(pool);
 
 	if (likely(worker))
 		wake_up_process(worker->task);
+
+	rt_unlock_idle_list(pool);
 }
 
 /**
@@ -839,7 +875,7 @@ void wq_worker_running(struct task_struct *task)
  */
 void wq_worker_sleeping(struct task_struct *task)
 {
-	struct worker *next, *worker = kthread_data(task);
+	struct worker *worker = kthread_data(task);
 	struct worker_pool *pool;
 
 	/*
@@ -856,25 +892,18 @@ void wq_worker_sleeping(struct task_struct *task)
 		return;
 
 	worker->sleeping = 1;
-	spin_lock_irq(&pool->lock);
+
 	/*
 	 * The counterpart of the following dec_and_test, implied mb,
 	 * worklist not empty test sequence is in insert_work().
 	 * Please read comment there.
-	 *
-	 * NOT_RUNNING is clear.  This means that we're bound to and
-	 * running on the local cpu w/ rq lock held and preemption
-	 * disabled, which in turn means that none else could be
-	 * manipulating idle_list, so dereferencing idle_list without pool
-	 * lock is safe.
 	 */
 	if (atomic_dec_and_test(&pool->nr_running) &&
 	    !list_empty(&pool->worklist)) {
-		next = first_worker(pool);
-		if (next)
-			wake_up_process(next->task);
+		sched_lock_idle_list(pool);
+		wake_up_worker(pool);
+		sched_unlock_idle_list(pool);
 	}
-	spin_unlock_irq(&pool->lock);
 }
 
 /**
@@ -1578,7 +1607,9 @@ static void worker_enter_idle(struct worker *worker)
 	worker->last_active = jiffies;
 
 	/* idle_list is LIFO */
+	rt_lock_idle_list(pool);
 	list_add(&worker->entry, &pool->idle_list);
+	rt_unlock_idle_list(pool);
 
 	if (too_many_workers(pool) && !timer_pending(&pool->idle_timer))
 		mod_timer(&pool->idle_timer, jiffies + IDLE_WORKER_TIMEOUT);
@@ -1611,7 +1642,9 @@ static void worker_leave_idle(struct worker *worker)
 		return;
 	worker_clr_flags(worker, WORKER_IDLE);
 	pool->nr_idle--;
+	rt_lock_idle_list(pool);
 	list_del_init(&worker->entry);
+	rt_unlock_idle_list(pool);
 }
 
 /**
@@ -1857,7 +1890,9 @@ static void destroy_worker(struct worker *worker)
 	 */
 	get_task_struct(worker->task);
 
+	rt_lock_idle_list(pool);
 	list_del_init(&worker->entry);
+	rt_unlock_idle_list(pool);
 	worker->flags |= WORKER_DIE;
 
 	idr_remove(&pool->worker_idr, worker->id);