Hi Oleg,
On Sunday 01 November 2015 20:05:00 Oleg Nesterov wrote:
> Hi Markus,
>
> Sorry again for delay. I was offlist. again.
Sorry I hadn't too much time last week.
>
> On 10/29, Markus Pargmann wrote:
> >
> > Hi,
> >
> > this is a try to remove all the signals from NBD. The first patch replaces the
> > signals. The other patches are some cleanups I made on the way.
> >
> > This should solve the kthread_run() problems as well.
>
> I obviously can't review these changes, I do not understand this code
> enough. But they look good imo.
Thanks for having a look.
>
> However, I do not understand the usage of ->task_recv and ->task_send.
>
> pid_show() doesn't even check nbd->task_recv != NULL. Honestly, I simply
> do not know if it can race with device_remove_file() or not. I think it
> can, but I can be easily wrong...
pid_show() should hopefully not be the problem. The 'pid' file which uses
pid_show() is created after task_recv was set and is removed using
device_remove_file(). Assuming that there are no open calls to pid_show() after
device_remove_file() was called this setup should not have a race issue.
>
> nbd_dbg_tasks_show() looks racy too even if it checks task_recv/task_send,
> at least this needs READ_ONCE() but in theory this is not enough,
> task_pid_nr() can read the freed task_struct.
Yes it requires a READ_ONCE(). But it is not possible for task_pid_nr to use
the freed task_struct. With the patches I posted, the send thread is kept alive
until kthread_stop() is called. The debugfs files are removed before the send
thread is stopped. So if there is a task struct it is valid.
>
> Again, I can easily miss something. But whatever I missed, perhaps the
> trivial (but uncompiled/untested) patch below makes sense anyway?
Yes as we don't need the task_struct anymore to send signals it makes sense.
Best Regards,
Markus
>
> Oleg.
>
>
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index f547005..67c1e09 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -63,8 +63,8 @@ struct nbd_device {
> struct timer_list timeout_timer;
> /* protects initialization and shutdown of the socket */
> spinlock_t sock_lock;
> - struct task_struct *task_recv;
> - struct task_struct *task_send;
> + pid_t task_recv;
> + pid_t task_send;
>
> #if IS_ENABLED(CONFIG_DEBUG_FS)
> struct dentry *dbg_dir;
> @@ -392,7 +392,7 @@ static ssize_t pid_show(struct device *dev,
> struct gendisk *disk = dev_to_disk(dev);
> struct nbd_device *nbd = (struct nbd_device *)disk->private_data;
>
> - return sprintf(buf, "%d\n", task_pid_nr(nbd->task_recv));
> + return sprintf(buf, "%d\n", nbd->task_recv);
> }
>
> static struct device_attribute pid_attr = {
> @@ -409,13 +409,13 @@ static int nbd_thread_recv(struct nbd_device *nbd)
>
> sk_set_memalloc(nbd->sock->sk);
>
> - nbd->task_recv = current;
> + nbd->task_recv = task_pid_nr(current);
>
> ret = device_create_file(disk_to_dev(nbd->disk), &pid_attr);
> if (ret) {
> dev_err(disk_to_dev(nbd->disk), "device_create_file failed!\n");
>
> - nbd->task_recv = NULL;
> + nbd->task_recv = 0;
>
> return ret;
> }
> @@ -432,7 +432,7 @@ static int nbd_thread_recv(struct nbd_device *nbd)
>
> device_remove_file(disk_to_dev(nbd->disk), &pid_attr);
>
> - nbd->task_recv = NULL;
> + nbd->task_recv = 0;
>
> return ret;
> }
> @@ -526,7 +526,7 @@ static int nbd_thread_send(void *data)
> struct nbd_device *nbd = data;
> struct request *req;
>
> - nbd->task_send = current;
> + nbd->task_send = task_pid_nr(current);
>
> set_user_nice(current, MIN_NICE);
> while (!kthread_should_stop() || !list_empty(&nbd->waiting_queue)) {
> @@ -549,7 +549,7 @@ static int nbd_thread_send(void *data)
> nbd_handle_req(nbd, req);
> }
>
> - nbd->task_send = NULL;
> + nbd->task_send = 0;
>
> return 0;
> }
> @@ -827,9 +827,9 @@ static int nbd_dbg_tasks_show(struct seq_file *s, void *unused)
> struct nbd_device *nbd = s->private;
>
> if (nbd->task_recv)
> - seq_printf(s, "recv: %d\n", task_pid_nr(nbd->task_recv));
> + seq_printf(s, "recv: %d\n", nbd->task_recv);
> if (nbd->task_send)
> - seq_printf(s, "send: %d\n", task_pid_nr(nbd->task_send));
> + seq_printf(s, "send: %d\n", nbd->task_send);
>
> return 0;
> }
>
>
--
Pengutronix e.K. | |
Industrial Linux Solutions | http://www.pengutronix.de/ |
Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 |
Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |
Attachment:
signature.asc
Description: This is a digitally signed message part.