netdev - Re: [PATCH] wwan_hwsim: Avoid flush_scheduled

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Date:   Wed, 20 Apr 2022 11:53:07 +0200
From:   Loic Poulain <loic.poulain@...aro.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Sergey Ryazanov <ryazanov.s.a@...il.com>,
        Johannes Berg <johannes@...solutions.net>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH] wwan_hwsim: Avoid flush_scheduled_work() usage

Hi Tetsuo, Sergey,

On Wed, 20 Apr 2022 at 04:22, Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
>
> Flushing system-wide workqueues is dangerous and will be forbidden.
> Replace system_wq with local wwan_wq.
>
> Link: https://lkml.kernel.org/r/49925af7-78a8-a3dd-bce6-cfc02e1a9236@I-love.SAKURA.ne.jp
> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>

Could you add a 'Fixes' tag?

> ---
> Note: This patch is only compile tested. By the way, don't you want to call
> debugfs_remove(wwan_hwsim_debugfs_devcreate) at err_clean_devs label in
> wwan_hwsim_init() like wwan_hwsim_exit() does, for debugfs_create_file("devcreate")
> is called before "goto err_clean_devs" happens?
>
>  drivers/net/wwan/wwan_hwsim.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/net/wwan/wwan_hwsim.c b/drivers/net/wwan/wwan_hwsim.c
> index 5b62cf3b3c42..2136319f588f 100644
> --- a/drivers/net/wwan/wwan_hwsim.c
> +++ b/drivers/net/wwan/wwan_hwsim.c
> @@ -33,6 +33,7 @@ static struct dentry *wwan_hwsim_debugfs_devcreate;
>  static DEFINE_SPINLOCK(wwan_hwsim_devs_lock);
>  static LIST_HEAD(wwan_hwsim_devs);
>  static unsigned int wwan_hwsim_dev_idx;
> +static struct workqueue_struct *wwan_wq;
>
>  struct wwan_hwsim_dev {
>         struct list_head list;
> @@ -371,7 +372,7 @@ static ssize_t wwan_hwsim_debugfs_portdestroy_write(struct file *file,
>          * waiting this callback to finish in the debugfs_remove() call. So,
>          * use workqueue.
>          */
> -       schedule_work(&port->del_work);
> +       queue_work(wwan_wq, &port->del_work);
>
>         return count;
>  }
> @@ -416,7 +417,7 @@ static ssize_t wwan_hwsim_debugfs_devdestroy_write(struct file *file,
>          * waiting this callback to finish in the debugfs_remove() call. So,
>          * use workqueue.
>          */
> -       schedule_work(&dev->del_work);
> +       queue_work(wwan_wq, &dev->del_work);
>
>         return count;
>  }
> @@ -506,9 +507,15 @@ static int __init wwan_hwsim_init(void)
>         if (wwan_hwsim_devsnum < 0 || wwan_hwsim_devsnum > 128)
>                 return -EINVAL;
>
> +       wwan_wq = alloc_workqueue("wwan_wq", 0, 0);
> +       if (!wwan_wq)
> +               return -ENOMEM;
> +
>         wwan_hwsim_class = class_create(THIS_MODULE, "wwan_hwsim");
> -       if (IS_ERR(wwan_hwsim_class))
> +       if (IS_ERR(wwan_hwsim_class)) {
> +               destroy_workqueue(wwan_wq);
>                 return PTR_ERR(wwan_hwsim_class);
> +       }
>
>         wwan_hwsim_debugfs_topdir = debugfs_create_dir("wwan_hwsim", NULL);
>         wwan_hwsim_debugfs_devcreate =
> @@ -524,6 +531,7 @@ static int __init wwan_hwsim_init(void)
>
>  err_clean_devs:
>         wwan_hwsim_free_devs();
> +       destroy_workqueue(wwan_wq);
>         debugfs_remove(wwan_hwsim_debugfs_topdir);
>         class_destroy(wwan_hwsim_class);
>
> @@ -534,7 +542,7 @@ static void __exit wwan_hwsim_exit(void)
>  {
>         debugfs_remove(wwan_hwsim_debugfs_devcreate);   /* Avoid new devs */
>         wwan_hwsim_free_devs();
> -       flush_scheduled_work();         /* Wait deletion works completion */
> +       destroy_workqueue(wwan_wq);             /* Wait deletion works completion */

Wouldn't it be simpler to just remove the flush call. It Looks like
all ports have been removed at that point, and all works cancelled,
right?

>         debugfs_remove(wwan_hwsim_debugfs_topdir);
>         class_destroy(wwan_hwsim_class);
>  }
> --
> 2.32.0

Regards,
Loic