workqueue: Fix PF_THREAD_BOUND abuse

PF_THREAD_BOUND is set by kthread_bind() and means the thread is bound
to a particular cpu for correctness. The workqueue code abuses this
flag and blindly sets it for all created threads, including those that
are free to migrate.

Restore the original semantics now that the worst abuse in the
cpu-hotplug path are gone. The only icky bit is the rescue thread for
per-cpu workqueues, this cannot use kthread_bind() but will use
set_cpus_allowed_ptr() to migrate itself to the desired cpu.

Set and clear PF_THREAD_BOUND manually here.

XXX: I think worker_maybe_bind_and_lock()/worker_unbind_and_unlock()
should also do a get_online_cpus(), this would likely allow us to
remove the while loop.

XXX: should probably repurpose GCWQ_DISASSOCIATED to warn on adding
works after CPU_DOWN_PREPARE -- its dual use to mark unbound gcwqs is
a tad annoying though.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 20 insertions, 9 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index ba977c4d8d4f..dc50d5d42428 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1283,8 +1283,14 @@ __acquires(&gcwq->lock)
 			return false;
 		if (task_cpu(task) == gcwq->cpu &&
 		    cpumask_equal(&current->cpus_allowed,
-				  get_cpu_mask(gcwq->cpu)))
+				  get_cpu_mask(gcwq->cpu))) {
+			/*
+			 * Since we're binding to a particular cpu and need to
+			 * stay there for correctness, mark us PF_THREAD_BOUND.
+			 */
+			task->flags |= PF_THREAD_BOUND;
 			return true;
+		}
 		spin_unlock_irq(&gcwq->lock);
 
 		/*
@@ -1298,6 +1304,18 @@ __acquires(&gcwq->lock)
 	}
 }
 
+static void worker_unbind_and_unlock(struct worker *worker)
+{
+	struct global_cwq *gcwq = worker->gcwq;
+	struct task_struct *task = worker->task;
+
+	/*
+	 * Its no longer required we're PF_THREAD_BOUND, the work is done.
+	 */
+	task->flags &= ~PF_THREAD_BOUND;
+	spin_unlock_irq(&gcwq->lock);
+}
+
 static struct worker *alloc_worker(void)
 {
 	struct worker *worker;
@@ -1360,15 +1378,9 @@ static struct worker *create_worker(struct global_cwq *gcwq, bool bind)
 	if (IS_ERR(worker->task))
 		goto fail;
 
-	/*
-	 * A rogue worker will become a regular one if CPU comes
-	 * online later on.  Make sure every worker has
-	 * PF_THREAD_BOUND set.
-	 */
 	if (bind && !on_unbound_cpu)
 		kthread_bind(worker->task, gcwq->cpu);
 	else {
-		worker->task->flags |= PF_THREAD_BOUND;
 		if (on_unbound_cpu)
 			worker->flags |= WORKER_UNBOUND;
 	}
@@ -2045,7 +2057,7 @@ repeat:
 		if (keep_working(gcwq))
 			wake_up_worker(gcwq);
 
-		spin_unlock_irq(&gcwq->lock);
+		worker_unbind_and_unlock(rescuer);
 	}
 
 	schedule();
@@ -2995,7 +3007,6 @@ struct workqueue_struct *__alloc_workqueue_key(const char *fmt,
 		if (IS_ERR(rescuer->task))
 			goto err;
 
-		rescuer->task->flags |= PF_THREAD_BOUND;
 		wake_up_process(rescuer->task);
 	}