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    SubjectRe: [PATCH v2 4/5]nbd: make nbd device wait for its users.
    Hey Markus,

    On Wed, Jun 15, 2016 at 12:00 PM, Markus Pargmann <mpa@pengutronix.de> wrote:
    > Hi Pranay,
    >
    > On Tuesday 14 June 2016 15:03:40 Pranay Srivastava wrote:
    >> 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.
    >
    > Thanks for the explanations. I think my understanding was off by one ;).
    > I didn't realize that the DO_IT thread from the userspace has the block
    > device open as well.
    >
    > I thought a bit about this, does it make sense to delay the essential
    > cleanup steps until really all open file handles were closed? So that
    > even if the DO_IT thread exits, the block device is still there. Only if
    > the file is closed everything is cleaned up. Maybe this makes the code
    > simpler and we can directly use krefs without any strange constructs.
    > What do you think?
    >

    I chose open/close as that is the common interface to all processes that need
    to use nbd device and not just nbd-client.

    Let's take example of a mount, so some user has just done a mount of this device
    and right now it's not doing anything. So someone issues an NBD_DISCONNECT
    and wham... The idea is to propagate errors to user space correctly.

    So the solution I've proposed says, if there's someone
    apart_from_nbd-client (which
    is currently waiting for NBD_DO_IT to complete) is
    using this device then nbd-client should honor that and shouldn't
    allow this device
    to be reused until after all such processes have left this device.


    > This would also allow the client to setup a new socket as long as it
    > does not close the nbd file handle.

    I think that is still possible right, which is why there's a kref_sub
    of 2, so the wait
    is only for the "other processes" using this device and _not_ this
    nbd-client whose
    NBD_DO_IT just got over.

    Now I'm just concerned over the processes which are trying to use this
    nbd device
    but the re-initialzation code at the end of NBD_DO_IT hasn't been done. So it's
    possible some device may skip getting a kref, due to kref_get_unless_zero.

    Actually I wanted to put this kref under bd_mutex to avoid such races and it'll
    always be a call to kref_get in open and kref_put in close. Still
    kref_sub(2) would
    be required :-)

    >
    > Could this behavior be potentially problematic for any client
    > implementation? Does it solve our other issue with setting up a new
    > sockets for an existing nbd blockdevice?

    I don't think that would be a problem. sock_shutdown is before the wait
    right? So in error condition the worker thread would close it and set to null
    while for a normal case, sock_shutdown in NBD_DO_IT would set that.
    So it should be OK.

    >
    > Cc Wouter
    >
    > Best Regards,
    >
    > Markus
    >
    >>
    >>
    >> >
    >> > 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);
    >> > > }
    >> > >
    >> >
    >> > --
    >> > 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 |
    >>
    >>
    >>
    >>
    >>
    >
    > --
    > 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 |



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    ---P.K.S

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