Re: [PATCH v2 4/5]nbd: make nbd device wait for its users.

[Pranay Srivastava <pranjas@gmail.com>]

Hi Markus,

On Tue, Jun 14, 2016 at 2:29 PM, Markus Pargmann <mpa@pengutronix.de> wrote:
>
> On Thursday 02 June 2016 13:25:00 Pranay Kr. Srivastava wrote:
> > When a timeout occurs or a recv fails, then
> > instead of abruplty killing nbd block device
> > wait for it's users to finish.
> >
> > This is more required when filesystem(s) like
> > ext2 or ext3 don't expect their buffer heads to
> > disappear while the filesystem is mounted.
> >
> > Each open of a nbd device is refcounted, while
> > the userland program [nbd-client] doing the
> > NBD_DO_IT ioctl would now wait for any other users
> > of this device before invalidating the nbd device.
> >
> > Signed-off-by: Pranay Kr. Srivastava <pranjas@gmail.com>
> > ---
> >  drivers/block/nbd.c | 58 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 58 insertions(+)
> >
> > diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> > index d1d898d..4da40dc 100644
> > --- a/drivers/block/nbd.c
> > +++ b/drivers/block/nbd.c
> > @@ -70,10 +70,13 @@ struct nbd_device {
> >  #if IS_ENABLED(CONFIG_DEBUG_FS)
> >       struct dentry *dbg_dir;
> >  #endif
> > +     atomic_t inuse;
> >       /*
> >        *This is specifically for calling sock_shutdown, for now.
> >        */
> >       struct work_struct ws_shutdown;
> > +     struct kref users;
> > +     struct completion user_completion;
> >  };
> >
> >  #if IS_ENABLED(CONFIG_DEBUG_FS)
> > @@ -104,6 +107,7 @@ static DEFINE_SPINLOCK(nbd_lock);
> >   * Shutdown function for nbd_dev work struct.
> >   */
> >  static void nbd_ws_func_shutdown(struct work_struct *);
> > +static void nbd_kref_release(struct kref *);
> >
> >  static inline struct device *nbd_to_dev(struct nbd_device *nbd)
> >  {
> > @@ -682,6 +686,8 @@ static void nbd_reset(struct nbd_device *nbd)
> >       nbd->flags = 0;
> >       nbd->xmit_timeout = 0;
> >       INIT_WORK(&nbd->ws_shutdown, nbd_ws_func_shutdown);
> > +     init_completion(&nbd->user_completion);
> > +     kref_init(&nbd->users);
> >       queue_flag_clear_unlocked(QUEUE_FLAG_DISCARD, nbd->disk->queue);
> >       del_timer_sync(&nbd->timeout_timer);
> >  }
> > @@ -815,6 +821,14 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
> >               kthread_stop(thread);
> >
> >               sock_shutdown(nbd);
> > +             /*
> > +              * kref_init initializes with ref count as 1,
> > +              * nbd_client, or the user-land program executing
> > +              * this ioctl will make the refcount to 2[at least]
> > +              * so subtracting 2 from refcount.
> > +              */
> > +             kref_sub(&nbd->users, 2, nbd_kref_release);
>
> Why don't you use a kref_put?

Ok, so I'll try to explain as I've understood the problem.

When the module is loaded the kref is initialized to 1.

Suppose now, someone has started nbd-client [nbdC-1] , then this
nbd-client will increase the ref count to 2. So far so good...

Now let's say this device is being shutdown via nbd-client[nbdC-2].

nbdC-1 will subtract the refcount by two, it has to do in NBD_DO_IT
since device file will not
be closed until after ioctl is over, and it'll wait_for_completion.

nbdC-2 now closes it's use of device file, this makes the refcount as
zero and completion
is triggered with nbdC-1 completed.

Now we don't want to trigger kref_put when nbdC-1 closes the device
file so kref_put needs
to be conditional in this regard so for that in_use is used.


>
> > +             wait_for_completion(&nbd->user_completion);
> >               mutex_lock(&nbd->tx_lock);
> >               nbd_clear_que(nbd);
> >               kill_bdev(bdev);
> > @@ -865,13 +879,56 @@ static int nbd_ioctl(struct block_device *bdev, fmode_t mode,
> >
> >       return error;
> >  }
> > +static void nbd_kref_release(struct kref *kref_users)
> > +{
> > +     struct nbd_device *nbd = container_of(kref_users, struct nbd_device,
> > +                     users);
>
> Not indented to opening bracket.
>
> > +     pr_debug("Releasing kref [%s]\n", __func__);
> > +     atomic_set(&nbd->inuse, 0);
> > +     complete(&nbd->user_completion);
> > +
> > +}
> > +
> > +static int nbd_open(struct block_device *bdev, fmode_t mode)
> > +{
> > +     struct nbd_device *nbd_dev = bdev->bd_disk->private_data;
> > +
> > +     if (kref_get_unless_zero(&nbd_dev->users))
> > +             atomic_set(&nbd_dev->inuse, 1);
> > +
> > +     pr_debug("Opening nbd_dev %s. Active users = %u\n",
> > +                     bdev->bd_disk->disk_name,
> > +                     atomic_read(&nbd_dev->users.refcount) - 1);
>
> Indent to opening bracket.
>
> > +     return 0;
> > +}
> > +
> > +static void nbd_release(struct gendisk *disk, fmode_t mode)
> > +{
> > +     struct nbd_device *nbd_dev = disk->private_data;
> > +     /*
> > +     *kref_init initializes ref count to 1, so we
> > +     *we check for refcount to be 2 for a final put.
> > +     *
> > +     *kref needs to be re-initialized just here as the
> > +     *other process holding it must see the ref count as 2.
> > +     */
> > +     if (atomic_read(&nbd_dev->inuse))
> > +             kref_put(&nbd_dev->users,  nbd_kref_release);
>

> What is this inuse atomic for? Everyone that releases the nbd device
> will need to execute a kref_put().

To do away with inuse, perhaps we can do

kref_get just before leaving the NBD_DO_IT? so that when device file
is closed everyone
would do a kref_put? However there's a small race window while the
kref is being initialized,
and another process [not just nbd-client] is trying to open the device.

Do you think it's better to do this by introducing a spin_lock instead
of atomic?

Let me know if my understanding is correct.


>
> Best Regards,
>
> Markus
>
> > +
> > +     pr_debug("Closing nbd_dev %s. Active users = %u\n",
> > +                     disk->disk_name,
> > +                     atomic_read(&nbd_dev->users.refcount) - 1);
> > +}
> >
> >  static const struct block_device_operations nbd_fops = {
> >       .owner =        THIS_MODULE,
> >       .ioctl =        nbd_ioctl,
> >       .compat_ioctl = nbd_ioctl,
> > +     .open =         nbd_open,
> > +     .release =      nbd_release
> >  };
> >
> > +
> >  static void nbd_ws_func_shutdown(struct work_struct *ws_nbd)
> >  {
> >       struct nbd_device *nbd_dev = container_of(ws_nbd, struct nbd_device,
> > @@ -1107,6 +1164,7 @@ static int __init nbd_init(void)
> >               disk->fops = &nbd_fops;
> >               disk->private_data = &nbd_dev[i];
> >               sprintf(disk->disk_name, "nbd%d", i);
> > +             atomic_set(&nbd_dev[i].inuse, 0);
> >               nbd_reset(&nbd_dev[i]);
> >               add_disk(disk);
> >       }
> >
>
> --
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