Re: [PATCH 01/13] driver core, libnvdimm: disable manual unbind of dimms while region active

[Dan Williams]

On Sat, Jun 4, 2016 at 2:10 PM, Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> On Sat, Jun 04, 2016 at 01:52:38PM -0700, Dan Williams wrote:
>> There are scenarios where we need a middle ground between disabling all
>> manual bind/unbind attempts (via driver->suppress_bind_attrs) and
>> allowing unbind at any userspace-determined time.  Pinning modules takes
>> away one vector for unwanted out-of-sequence device_release_driver()
>> invocations, this new mechanism (via device->suppress_unbind_attr) takes
>> away another.
>>
>> The first user of this mechanism is the libnvdimm sub-system where
>> manual dimm disabling should be prevented while the dimm is active in
>> any region.  Note that there is a 1:N dimm-to-region relationship which
>> is why this is implemented as a disable count rather than a flag.  This
>> forces userspace to disable regions before dimms when manually shutting
>> down a bus topology.
>>
>> This does not affect any of the kernel-internal initiated invocations of
>> device_release_driver().
>>
>> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>> ---
>>  drivers/base/base.h             |    1 +
>>  drivers/base/bus.c              |   12 ++++++++++--
>>  drivers/base/core.c             |    1 +
>>  drivers/base/dd.c               |    2 +-
>>  drivers/nvdimm/namespace_devs.c |    1 +
>>  drivers/nvdimm/region_devs.c    |    4 +++-
>>  include/linux/device.h          |   20 ++++++++++++++++++++
>>  7 files changed, 37 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/base/base.h b/drivers/base/base.h
>> index e05db388bd1c..129814b17ca6 100644
>> --- a/drivers/base/base.h
>> +++ b/drivers/base/base.h
>> @@ -109,6 +109,7 @@ extern int bus_add_driver(struct device_driver *drv);
>>  extern void bus_remove_driver(struct device_driver *drv);
>>
>>  extern void driver_detach(struct device_driver *drv);
>> +extern void __device_release_driver(struct device *dev);
>>  extern int driver_probe_device(struct device_driver *drv, struct device *dev);
>>  extern void driver_deferred_probe_del(struct device *dev);
>>  static inline int driver_match_device(struct device_driver *drv,
>> diff --git a/drivers/base/bus.c b/drivers/base/bus.c
>> index 6470eb8088f4..b48a903f2d28 100644
>> --- a/drivers/base/bus.c
>> +++ b/drivers/base/bus.c
>> @@ -188,10 +188,18 @@ static ssize_t unbind_store(struct device_driver *drv, const char *buf,
>>       if (dev && dev->driver == drv) {
>>               if (dev->parent)        /* Needed for USB */
>>                       device_lock(dev->parent);
>> -             device_release_driver(dev);
>> +
>> +             device_lock(dev);
>> +             if (atomic_read(&dev->suppress_unbind_attr) > 0)
>> +                     err = -EBUSY;
>> +             else {
>> +                     __device_release_driver(dev);
>> +                     err = count;
>> +             }
>> +             device_unlock(dev);
>> +
>>               if (dev->parent)
>>                       device_unlock(dev->parent);
>> -             err = count;
>>       }
>>       put_device(dev);
>>       bus_put(bus);
>> diff --git a/drivers/base/core.c b/drivers/base/core.c
>> index 0a8bdade53f2..0ea0e8560e1d 100644
>> --- a/drivers/base/core.c
>> +++ b/drivers/base/core.c
>> @@ -708,6 +708,7 @@ void device_initialize(struct device *dev)
>>       INIT_LIST_HEAD(&dev->devres_head);
>>       device_pm_init(dev);
>>       set_dev_node(dev, -1);
>> +     atomic_set(&dev->suppress_unbind_attr, 0);
>>  #ifdef CONFIG_GENERIC_MSI_IRQ
>>       INIT_LIST_HEAD(&dev->msi_list);
>>  #endif
>> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
>> index 16688f50729c..9e21817ca2d6 100644
>> --- a/drivers/base/dd.c
>> +++ b/drivers/base/dd.c
>> @@ -756,7 +756,7 @@ EXPORT_SYMBOL_GPL(driver_attach);
>>   * __device_release_driver() must be called with @dev lock held.
>>   * When called for a USB interface, @dev->parent lock must be held as well.
>>   */
>> -static void __device_release_driver(struct device *dev)
>> +void __device_release_driver(struct device *dev)
>>  {
>>       struct device_driver *drv;
>>
>> diff --git a/drivers/nvdimm/namespace_devs.c b/drivers/nvdimm/namespace_devs.c
>> index c5e3196c45b0..e65572b6092c 100644
>> --- a/drivers/nvdimm/namespace_devs.c
>> +++ b/drivers/nvdimm/namespace_devs.c
>> @@ -1950,6 +1950,7 @@ static int init_active_labels(struct nd_region *nd_region)
>>               }
>>               nd_mapping->ndd = ndd;
>>               atomic_inc(&nvdimm->busy);
>> +             device_disable_unbind_attr(&nvdimm->dev);
>>               get_ndd(ndd);
>>
>>               count = nd_label_active_count(ndd);
>> diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
>> index 40fcfea26fbb..320f0f3ea367 100644
>> --- a/drivers/nvdimm/region_devs.c
>> +++ b/drivers/nvdimm/region_devs.c
>> @@ -427,8 +427,10 @@ static void nd_region_notify_driver_action(struct nvdimm_bus *nvdimm_bus,
>>                       nd_mapping->labels = NULL;
>>                       put_ndd(ndd);
>>                       nd_mapping->ndd = NULL;
>> -                     if (ndd)
>> +                     if (ndd) {
>>                               atomic_dec(&nvdimm->busy);
>> +                             device_enable_unbind_attr(&nvdimm->dev);
>> +                     }
>>               }
>>
>>               if (is_nd_pmem(dev))
>> diff --git a/include/linux/device.h b/include/linux/device.h
>> index 38f02814d53a..d9eaa85bb9cf 100644
>> --- a/include/linux/device.h
>> +++ b/include/linux/device.h
>> @@ -849,6 +849,7 @@ struct device {
>>
>>       void    (*release)(struct device *dev);
>>       struct iommu_group      *iommu_group;
>> +     atomic_t                suppress_unbind_attr; /* disable manual unbind */
>>
>>       bool                    offline_disabled:1;
>>       bool                    offline:1;
>> @@ -988,6 +989,25 @@ static inline void device_lock_assert(struct device *dev)
>>       lockdep_assert_held(&dev->mutex);
>>  }
>>
>> +static inline bool device_disable_unbind_attr(struct device *dev)
>> +{
>> +     bool suppressed = false;
>> +
>> +     device_lock(dev);
>> +     if (dev->driver) {
>> +             atomic_inc(&dev->suppress_unbind_attr);
>> +             suppressed = true;
>> +     }
>> +     device_unlock(dev);
>> +
>> +     return suppressed;
>> +}
>> +
>> +static inline bool device_enable_unbind_attr(struct device *dev)
>> +{
>> +     return atomic_dec_and_test(&dev->suppress_unbind_attr);
>> +}
>> +
>
> Ick, that's a mess.
>
> Why not just prevent the unbinding from happening in your bus when you
> need it?

Because historically unbind never fails...

    void device_release_driver(struct device *dev);

> And as you are grabbing a lock, why is this an atomic variable?
>
> This is just making things _really_ complex for very limited gain for
> what I can tell.

I thought it was cleaner to have the disable action synchronize with
unbind, but the lock isn't needed to re-enable unbind.  I can move the
complexity to the caller to bounce the device_lock() and re-validate
that dev->driver is still set, but that does not seem cleaner.

If making device_release_driver() fail is an option I can take a look,
but that seems like a major change in policy.