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SubjectRe: [PATCH 2/2] kthread_worker: reimplement flush_kthread_work() to allow freeing the work item being executed
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On Thu, 2012-07-19 at 14:16 -0700, Tejun Heo wrote:
> From 06f9a06f4aeecdb9d07014713ab41b548ae219b5 Mon Sep 17 00:00:00 2001
> From: Tejun Heo <tj@kernel.org>
> Date: Thu, 19 Jul 2012 13:52:53 -0700
>
> kthread_worker provides minimalistic workqueue-like interface for
> users which need a dedicated worker thread (e.g. for realtime
> priority). It has basic queue, flush_work, flush_worker operations
> which mostly match the workqueue counterparts; however, due to the way
> flush_work() is implemented, it has a noticeable difference of not
> allowing work items to be freed while being executed.
>
> While the current users of kthread_worker are okay with the current
> behavior, the restriction does impede some valid use cases. Also,
> removing this difference isn't difficult and actually makes the code
> easier to understand.
>
> This patch reimplements flush_kthread_work() such that it uses a
> flush_work item instead of queue/done sequence numbers.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> ---
> include/linux/kthread.h | 8 +-----
> kernel/kthread.c | 48 ++++++++++++++++++++++++++--------------------
> 2 files changed, 29 insertions(+), 27 deletions(-)

Hi Tejun,

I have a question and comment below.

> diff --git a/include/linux/kthread.h b/include/linux/kthread.h
> index 0714b24..22ccf9d 100644
> --- a/include/linux/kthread.h
> +++ b/include/linux/kthread.h
> @@ -49,8 +49,6 @@ extern int tsk_fork_get_node(struct task_struct *tsk);
> * can be queued and flushed using queue/flush_kthread_work()
> * respectively. Queued kthread_works are processed by a kthread
> * running kthread_worker_fn().
> - *
> - * A kthread_work can't be freed while it is executing.
> */
> struct kthread_work;
> typedef void (*kthread_work_func_t)(struct kthread_work *work);
> @@ -59,15 +57,14 @@ struct kthread_worker {
> spinlock_t lock;
> struct list_head work_list;
> struct task_struct *task;
> + struct kthread_work *current_work;
> };
>
> struct kthread_work {
> struct list_head node;
> kthread_work_func_t func;
> wait_queue_head_t done;
> - atomic_t flushing;
> - int queue_seq;
> - int done_seq;
> + struct kthread_worker *worker;
> };
>
> #define KTHREAD_WORKER_INIT(worker) { \
> @@ -79,7 +76,6 @@ struct kthread_work {
> .node = LIST_HEAD_INIT((work).node), \
> .func = (fn), \
> .done = __WAIT_QUEUE_HEAD_INITIALIZER((work).done), \
> - .flushing = ATOMIC_INIT(0), \
> }
>
> #define DEFINE_KTHREAD_WORKER(worker) \
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 7b8a678..4034b2b 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -360,16 +360,12 @@ repeat:
> struct kthread_work, node);
> list_del_init(&work->node);
> }
> + worker->current_work = work;
> spin_unlock_irq(&worker->lock);
>
> if (work) {
> __set_current_state(TASK_RUNNING);
> work->func(work);

If the call to 'work->func(work);' frees the memory pointed to by
'work', 'worker->current_work' points to deallocated memory.
So 'worker->current_work' will only ever used as a unique 'work'
identifier to handle, correct?


> - smp_wmb(); /* wmb worker-b0 paired with flush-b1 */
> - work->done_seq = work->queue_seq;
> - smp_mb(); /* mb worker-b1 paired with flush-b0 */
> - if (atomic_read(&work->flushing))
> - wake_up_all(&work->done);
> } else if (!freezing(current))
> schedule();
>
> @@ -384,7 +380,7 @@ static void insert_kthread_work(struct kthread_worker *worker,
> struct list_head *pos)
> {
> list_add_tail(&work->node, pos);
> - work->queue_seq++;
> + work->worker = worker;
> if (likely(worker->task))
> wake_up_process(worker->task);
> }
> @@ -434,25 +430,35 @@ static void kthread_flush_work_fn(struct kthread_work *work)
> */
> void flush_kthread_work(struct kthread_work *work)
> {
> - int seq = work->queue_seq;
> + struct kthread_flush_work fwork = {
> + KTHREAD_WORK_INIT(fwork.work, kthread_flush_work_fn),
> + COMPLETION_INITIALIZER_ONSTACK(fwork.done),
> + };
> + struct kthread_worker *worker;
> + bool noop = false;
> +

You might want a check for 'work == NULL' here, to gracefully handle
code like the following:

void driver_work_handler(struct kthread_work *work)
{
...
kfree(work);
}
struct kthread_work *driver_queue_batch(void)
{
struct kthread_work *work = NULL;
...
while (driver_more_stuff_todo()) {
work = kzalloc(sizeof(struct kthread work), GFP_WHATEVER);
...
queue_kthread_work(&driver_worker, work);
}
return work;
}
void driver_foobar(void)
{
...
flush_kthread_work(driver_queue_batch());
...
}

Otherwise, things look OK to me.

Regards,
Andy

> +retry:
> + worker = work->worker;
> + if (!worker)
> + return;
>
> - atomic_inc(&work->flushing);
> + spin_lock_irq(&worker->lock);
> + if (work->worker != worker) {
> + spin_unlock_irq(&worker->lock);
> + goto retry;
> + }
>
> - /*
> - * mb flush-b0 paired with worker-b1, to make sure either
> - * worker sees the above increment or we see done_seq update.
> - */
> - smp_mb__after_atomic_inc();
> + if (!list_empty(&work->node))
> + insert_kthread_work(worker, &fwork.work, work->node.next);
> + else if (worker->current_work == work)
> + insert_kthread_work(worker, &fwork.work, worker->work_list.next);
> + else
> + noop = true;
>
> - /* A - B <= 0 tests whether B is in front of A regardless of overflow */
> - wait_event(work->done, seq - work->done_seq <= 0);
> - atomic_dec(&work->flushing);
> + spin_unlock_irq(&worker->lock);
>
> - /*
> - * rmb flush-b1 paired with worker-b0, to make sure our caller
> - * sees every change made by work->func().
> - */
> - smp_mb__after_atomic_dec();
> + if (!noop)
> + wait_for_completion(&fwork.done);
> }
> EXPORT_SYMBOL_GPL(flush_kthread_work);
>




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