linux-kernel - Re: [PATCH 1/3] nvmem: core: remove member users from struct nvmem

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Date:   Wed, 7 Jun 2017 23:51:05 +0200
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] nvmem: core: remove member users from struct
 nvmem_device

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.

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.
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.

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@...il.com>
>> ---
>>  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)
>>
>