[PATCH v7 06/10] kthread: Detect when a kthread work is used by more workers

[Petr Mladek]

Nothing currently prevents a work from queuing for a kthread worker
when it is already running on another one. This means that the work
might run in parallel on more workers. Also some operations, e.g.
flush or drain are not reliable.

This problem will be even more visible after we add cancel_kthread_work()
function. It will only have "work" as the parameter and will use
worker->lock to synchronize with others.

Well, normally this is not a problem because the API users are sane.
But bugs might happen and users also might be crazy.

This patch adds a warning when we try to insert the work for another
worker. It does not fully prevent the misuse because it would make the
code much more complicated without a big benefit.

It adds the same warning also into flush_kthread_work() instead of
the repeated attempts to get the right lock.

A side effect is that one needs to explicitly reinitialize the work
if it must be queued into another worker. This is needed, for example,
when the worker is stopped and started again. It is a bit inconvenient.
But it looks like a good compromise between the stability and complexity.

I have double checked all existing users of the kthread worker API
and they all seems to initialize the work after the worker gets
started.

Just for completeness, the patch adds a check that the work is not
already in a queue.

The patch also puts all the checks into a separate function. It will
be reused when implementing delayed works.

Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
---
 kernel/kthread.c | 28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 441651765f08..385a7d6b4872 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -574,6 +574,9 @@ EXPORT_SYMBOL_GPL(__init_kthread_worker);
  * The works are not allowed to keep any locks, disable preemption or interrupts
  * when they finish. There is defined a safe point for freezing when one work
  * finishes and before a new one is started.
+ *
+ * Also the works must not be handled by more workers at the same time, see also
+ * queue_kthread_work().
  */
 int kthread_worker_fn(void *worker_ptr)
 {
@@ -710,12 +713,21 @@ create_kthread_worker_on_cpu(int cpu, const char namefmt[], ...)
 }
 EXPORT_SYMBOL(create_kthread_worker_on_cpu);
 
+static void insert_kthread_work_sanity_check(struct kthread_worker *worker,
+					       struct kthread_work *work)
+{
+	lockdep_assert_held(&worker->lock);
+	WARN_ON_ONCE(!list_empty(&work->node));
+	/* Do not use a work with more workers, see queue_kthread_work() */
+	WARN_ON_ONCE(work->worker && work->worker != worker);
+}
+
 /* insert @work before @pos in @worker */
 static void insert_kthread_work(struct kthread_worker *worker,
-			       struct kthread_work *work,
-			       struct list_head *pos)
+				struct kthread_work *work,
+				struct list_head *pos)
 {
-	lockdep_assert_held(&worker->lock);
+	insert_kthread_work_sanity_check(worker, work);
 
 	list_add_tail(&work->node, pos);
 	work->worker = worker;
@@ -731,6 +743,9 @@ static void insert_kthread_work(struct kthread_worker *worker,
  * Queue @work to work processor @task for async execution.  @task
  * must have been created with kthread_worker_create().  Returns %true
  * if @work was successfully queued, %false if it was already pending.
+ *
+ * Reinitialize the work if it needs to be used by another worker.
+ * For example, when the worker was stopped and started again.
  */
 bool queue_kthread_work(struct kthread_worker *worker,
 			struct kthread_work *work)
@@ -775,16 +790,13 @@ void flush_kthread_work(struct kthread_work *work)
 	struct kthread_worker *worker;
 	bool noop = false;
 
-retry:
 	worker = work->worker;
 	if (!worker)
 		return;
 
 	spin_lock_irq(&worker->lock);
-	if (work->worker != worker) {
-		spin_unlock_irq(&worker->lock);
-		goto retry;
-	}
+	/* Work must not be used with more workers, see queue_kthread_work(). */
+	WARN_ON_ONCE(work->worker != worker);
 
 	if (!list_empty(&work->node))
 		insert_kthread_work(worker, &fwork.work, work->node.next);
-- 
1.8.5.6