Re: [PATCH v4 3/5]nbd: make nbd device wait for its users

[Pranay Srivastava <pranjas@gmail.com>]

On Sun, Jul 10, 2016 at 6:32 PM, Markus Pargmann <mpa@pengutronix.de> wrote:
> On 2016 M06 30, Thu 14:02:03 CEST Pranay Kr. Srivastava wrote:
>> When a timeout occurs or a recv fails, then
>> instead of abruplty killing nbd block device
>> wait for its 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.
>>
>> A timedout or a disconnected device, if in use, can't
>> be used until it has been resetted. The reset happens
>> when all tasks having this bdev open closes this bdev.
>>
>> Signed-off-by: Pranay Kr. Srivastava <pranjas@gmail.com>
>> ---
>>  drivers/block/nbd.c | 106
>> ++++++++++++++++++++++++++++++++++++++++++---------- 1 file changed, 87
>> insertions(+), 19 deletions(-)
>>
>> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
>> index e362d44..fb56dd2 100644
>> --- a/drivers/block/nbd.c
>> +++ b/drivers/block/nbd.c
>> @@ -72,6 +72,8 @@ struct nbd_device {
>>  #endif
>>       /* 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)
>> @@ -99,6 +101,8 @@ static int max_part;
>>  static DEFINE_SPINLOCK(nbd_lock);
>>
>>  static void nbd_ws_func_shutdown(struct work_struct *);
>> +static void nbd_kref_release(struct kref *);
>> +static int nbd_size_clear(struct nbd_device *, struct block_device *);
>
> More function signatures. Why?

To avoid code move. But do let me know why is code signature(s)
like this are bad , just asking to avoid such things.

>
>>
>>  static inline struct device *nbd_to_dev(struct nbd_device *nbd)
>>  {
>> @@ -145,11 +149,9 @@ static int nbd_size_set(struct nbd_device *nbd, struct
>> block_device *bdev, int blocksize, int nr_blocks)
>>  {
>>       int ret;
>> -
>>       ret = set_blocksize(bdev, blocksize);
>>       if (ret)
>>               return ret;
>> -
>
> Unrelated.
>
>>       nbd->blksize = blocksize;
>>       nbd->bytesize = (loff_t)blocksize * (loff_t)nr_blocks;
>>
>> @@ -197,6 +199,9 @@ static void nbd_xmit_timeout(unsigned long arg)
>>  {
>>       struct nbd_device *nbd = (struct nbd_device *)arg;
>>
>> +     if (nbd->timedout)
>> +             return;
>> +
>
> What does this have to do with the patch?

to avoid re-scheduling the work function. Apparently that did
cause some trouble with ext4 and 10K dd processes.

>
>>       if (list_empty(&nbd->queue_head))
>>               return;
>>
>> @@ -472,8 +477,6 @@ static int nbd_thread_recv(struct nbd_device *nbd,
>> struct block_device *bdev) nbd_end_request(nbd, req);
>>       }
>>
>> -     nbd_size_clear(nbd, bdev);
>> -
>>       device_remove_file(disk_to_dev(nbd->disk), &dev_attr_pid);
>>
>>       nbd->task_recv = NULL;
>> @@ -650,12 +653,13 @@ static int nbd_set_socket(struct nbd_device *nbd,
>> struct socket *sock) int ret = 0;
>>
>>       spin_lock(&nbd->sock_lock);
>> -     if (nbd->sock)
>> +
>> +     if (nbd->sock || nbd->timedout)
>>               ret = -EBUSY;
>
> nbd->timedout is already checked in __nbd_ioctl(), no need to check it twice.
>
>>       else
>>               nbd->sock = sock;
>> -     spin_unlock(&nbd->sock_lock);
>>
>> +     spin_unlock(&nbd->sock_lock);
>
> random modification.
>
>>       return ret;
>>  }
>>
>> @@ -670,6 +674,7 @@ 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);
>>       queue_flag_clear_unlocked(QUEUE_FLAG_DISCARD, nbd->disk->queue);
>>       del_timer_sync(&nbd->timeout_timer);
>>  }
>> @@ -704,6 +709,9 @@ static void nbd_dev_dbg_close(struct nbd_device *nbd);
>>  static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
>>                      unsigned int cmd, unsigned long arg)
>>  {
>> +     if (nbd->timedout || nbd->disconnect)
>> +             return -EBUSY;
>> +
>>       switch (cmd) {
>>       case NBD_DISCONNECT: {
>>               struct request sreq;
>> @@ -733,7 +741,6 @@ static int __nbd_ioctl(struct block_device *bdev, struct
>> nbd_device *nbd, nbd_clear_que(nbd);
>>               BUG_ON(!list_empty(&nbd->queue_head));
>>               BUG_ON(!list_empty(&nbd->waiting_queue));
>> -             kill_bdev(bdev);
>>               return 0;
>>
>>       case NBD_SET_SOCK: {
>> @@ -752,7 +759,6 @@ static int __nbd_ioctl(struct block_device *bdev, struct
>> nbd_device *nbd,
>>
>>       case NBD_SET_BLKSIZE: {
>>               loff_t bsize = div_s64(nbd->bytesize, arg);
>> -
>
> random modification.
>
>>               return nbd_size_set(nbd, bdev, arg, bsize);
>>       }
>>
>> @@ -804,22 +810,29 @@ static int __nbd_ioctl(struct block_device *bdev,
>> struct nbd_device *nbd, error = nbd_thread_recv(nbd, bdev);
>>               nbd_dev_dbg_close(nbd);
>>               kthread_stop(thread);
>> -             sock_shutdown(nbd);
>> -
>> -             mutex_lock(&nbd->tx_lock);
>> -             nbd->task_recv = NULL;
>>
>> -             nbd_clear_que(nbd);
>> -             kill_bdev(bdev);
>> -             nbd_bdev_reset(bdev);
>> +             sock_shutdown(nbd);
>>
>>               if (nbd->disconnect) /* user requested, ignore socket errors */
>>                       error = 0;
>>               if (nbd->timedout)
>>                       error = -ETIMEDOUT;
>>
>> -             nbd_reset(nbd);
>> +             mutex_lock(&nbd->tx_lock);
>> +             nbd_clear_que(nbd);
>> +             nbd->disconnect = true; /* To kill bdev*/
>> +             mutex_unlock(&nbd->tx_lock);
>> +             cancel_work_sync(&nbd->ws_shutdown);
>> +             kref_put(&nbd->users, nbd_kref_release);
>> +             wait_for_completion(&nbd->user_completion);
>>
>> +             mutex_lock(&bdev->bd_mutex);
>> +             if (!kref_get_unless_zero(&nbd->users))
>> +                     kref_init(&nbd->users);
>
> This kref usage simply looks wrong and confusing. I commented last time
> already
> that I think atomics will work better. Please discuss with me what you think
> before sending out a new version. Otherwise this patch series will increase in
> version forever.

Alright let's go with atomics.
But why this looks wrong, are you referring to partitioned device?

>
>> +             mutex_unlock(&bdev->bd_mutex);
>> +
>> +             mutex_lock(&nbd->tx_lock);
>> +             nbd_reset(nbd);
>>               return error;
>>       }
>>
>> @@ -857,19 +870,74 @@ 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
>> +                                             );
>> +     schedule_work(&nbd->ws_shutdown);
>
> Do we need to schedule work here?

Yes this is for the kill_bdev part. This is the final kick to bdev which happens
after the wait in NBD_DO_IT.

>
>> +}
>> +
>> +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))
>> +             kref_init(&nbd_dev->users);
>> +
>> +     pr_debug("Opening nbd_dev %s. Active users = %u\n",
>> +                     bdev->bd_disk->disk_name,
>> +                     atomic_read(&nbd_dev->users.refcount)
>> +             );
>> +     return 0;
>> +}
>> +
>> +static void nbd_release(struct gendisk *disk, fmode_t mode)
>> +{
>> +     struct nbd_device *nbd_dev = disk->private_data;
>> +
>> +     kref_put(&nbd_dev->users,  nbd_kref_release);
>> +
>> +     pr_debug("Closing nbd_dev %s. Active users = %u\n",
>> +                     disk->disk_name,
>> +                     atomic_read(&nbd_dev->users.refcount)
>> +             );
>> +}
>>
>>  static const struct block_device_operations nbd_fops = {
>>       .owner =        THIS_MODULE,
>>       .ioctl =        nbd_ioctl,
>>       .compat_ioctl = nbd_ioctl,
>> +     .open =         nbd_open,
>> +     .release =      nbd_release
>>  };
>>
>> +
>
> random modification
>
>>  static void nbd_ws_func_shutdown(struct work_struct *ws_nbd)
>>  {
>>       struct nbd_device *nbd_dev = container_of(ws_nbd, struct nbd_device,
>> -                     ws_shutdown);
>> -
>> -     sock_shutdown(nbd_dev);
>> +                                                     ws_shutdown
>> +                                             );
>
> ...???

Tried to match the brackets... that's what you meant earlier?

>
>> +
>> +     struct block_device *bdev = bdget(part_devt(
>> +                                             dev_to_part(nbd_to_dev(nbd_dev))
>> +                                             )
>> +                                     );
>> +     BUG_ON(!bdev);
>
> A simple check would be enough. Or a warning.

Ok, but that's really a bug.

>
>> +     if (nbd_dev->timedout)
>> +             sock_shutdown(nbd_dev);
>
> This timeout check seems unnecessary. If we do not timeout and the socket was
> already closed, the sock_shutdown() will do nothing.
>
>
> So if I understand you correctly you are trying to block all ioctls while you
> are shutting down which is a well a behaviour change of the ioctl interface.
> Why do you think it is better not to allow any changes until everyone closed
> the blockdevice? Shouldn't there be some control left for the user, for
> example
> CLEAR_SOCK?

Ah... Yes that's indeed what I'm trying to do. Now say if this block
device is mounted
and another nbd-client is trying to disconnect it [CLEAR + DISCONNECT]
then clear
is doing a kill_bdev. Socket already has been disconnected but the
device is just not
usable in this case.

If however we are trying to provide for an error recovery, like live
mounted device
and there's was timeout with all connections teared down and then someone does
a set socket on this? Is this supported currently ?

A change in the CLEAR, like not actually killing bdev would also not be good. So
better avoid such ioctl if device is in use, no?

>
> Regards,
>
> Markus
>
>> +
>> +     if (nbd_dev->disconnect) {
>> +             mutex_lock(&nbd_dev->tx_lock);
>> +             nbd_dev->task_recv = NULL;
>> +             nbd_clear_que(nbd_dev);
>> +             kill_bdev(bdev);
>> +             nbd_bdev_reset(bdev);
>> +             mutex_unlock(&nbd_dev->tx_lock);
>> +             nbd_size_clear(nbd_dev, bdev);
>> +             complete(&nbd_dev->user_completion);
>> +     }
>> +     bdput(bdev);
>>  }
>>
>>  #if IS_ENABLED(CONFIG_DEBUG_FS)
>
>



-- 
        ---P.K.S