Re: [PATCH 1/3] nvmem: core: remove member users from struct nvmem_device

[Srinivas Kandagatla]

On 07/06/17 22:51, Heiner Kallweit wrote:
> Am 07.06.2017 um 17:30 schrieb Srinivas Kandagatla:
> > On 04/06/17 12:01, Heiner Kallweit wrote:
> > > Member users is used only to check whether we're allowed to remove
> > > the module. So in case of built-in it's not used at all and in case
> >
> > nvmem providers doesn't have to be independent drivers, providers could be part of the other driver which can dynamically register and unregister nvmem providers. For example at24 and at25 drivers.
> >
> > This patch will break such cases !!
> Thanks for the quick review.
> I don't think this patch breaks e.g. at24 / at25. Let me try to explain:
>
> at24 / at25 set themself as owner in struct nvmem_device and nvmem_unregister
> is called from at24/25_remove only. These remove callbacks are called only if
> all references to the respective module have been released.
>
> In current kernel code I don't see any nvmem use broken by the proposed patch.
> However in general you're right, there may be future use cases where
> nvmem_unregister isn't called only from a remove callback.

Yes, the patch would not break the exiting code, but as said it would 
break a feature which was considered while writing the code.

> If the refcount isn't zero when calling nvmem_unregister then there's a bigger
> problem, I don't think there's any normal use case where this can happen.

Yes I understand chances of this error path is slim but it would crash 
the system if it hits this path, so this safety check is in place.

> Instead of just returning -EBUSY I think a WARN() would be appropriate.
> Currently no caller of nvmem_unregister checks the return code anyway.
> My opinion would be that the refcount here is more a debug feature.
>
>
> Whilst we're talking about nvmem_unregister:
> I think the device_del() at the end should be a device_unregister().
> Else we miss put_device as second part of destroying a device.

These issues have already been addressed in
https://patchwork.kernel.org/patch/9685559/
https://patchwork.kernel.org/patch/9685561/
https://patchwork.kernel.org/patch/9729235/


--srini

> Rgds, Heiner
>
>
> > > that owner is a module we have the module refcount for the same
> > > purpose already. Whenever users is incremented the owner's refcount
> > > is incremented too. Therefore users isn't needed.
> > >
> > > Signed-off-by: Heiner Kallweit <hkallweit1@xxxxxxxxx>
> > > ---
> > >   drivers/nvmem/core.c | 16 ----------------
> > >   1 file changed, 16 deletions(-)
> > >
> > > diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
> > > index 8c830a80..4e07f3f8 100644
> > > --- a/drivers/nvmem/core.c
> > > +++ b/drivers/nvmem/core.c
> > > @@ -33,7 +33,6 @@ struct nvmem_device {
> > >       int            word_size;
> > >       int            ncells;
> > >       int            id;
> > > -    int            users;
> > >       size_t            size;
> > >       bool            read_only;
> > >       int            flags;
> > > @@ -517,13 +516,6 @@ EXPORT_SYMBOL_GPL(nvmem_register);
> > >    */
> > >   int nvmem_unregister(struct nvmem_device *nvmem)
> > >   {
> > > -    mutex_lock(&nvmem_mutex);
> > > -    if (nvmem->users) {
> > > -        mutex_unlock(&nvmem_mutex);
> > > -        return -EBUSY;
> > > -    }
> > > -    mutex_unlock(&nvmem_mutex);
> > > -
> > >       if (nvmem->flags & FLAG_COMPAT)
> > >           device_remove_bin_file(nvmem->base_dev, &nvmem->eeprom);
> > >
> > > @@ -562,7 +554,6 @@ static struct nvmem_device *__nvmem_device_get(struct device_node *np,
> > >           }
> > >       }
> > >
> > > -    nvmem->users++;
> > >       mutex_unlock(&nvmem_mutex);
> > >
> > >       if (!try_module_get(nvmem->owner)) {
> > > @@ -570,10 +561,6 @@ static struct nvmem_device *__nvmem_device_get(struct device_node *np,
> > >               "could not increase module refcount for cell %s\n",
> > >               nvmem->name);
> > >
> > > -        mutex_lock(&nvmem_mutex);
> > > -        nvmem->users--;
> > > -        mutex_unlock(&nvmem_mutex);
> > > -
> > >           return ERR_PTR(-EINVAL);
> > >       }
> > >
> > > @@ -583,9 +570,6 @@ static struct nvmem_device *__nvmem_device_get(struct device_node *np,
> > >   static void __nvmem_device_put(struct nvmem_device *nvmem)
> > >   {
> > >       module_put(nvmem->owner);
> > > -    mutex_lock(&nvmem_mutex);
> > > -    nvmem->users--;
> > > -    mutex_unlock(&nvmem_mutex);
> > >   }
> > >
> > >   static int nvmem_match(struct device *dev, void *data)