Message-ID: <09b30741-b2eb-484a-bfe6-e1964d282e23@redhat.com>
Date: Tue, 9 Jan 2024 12:53:14 +0100
From: Marco Pagani <marpagan@...hat.com>
To: Xu Yilun <yilun.xu@...ux.intel.com>
Cc: Moritz Fischer <mdf@...nel.org>, Wu Hao <hao.wu@...el.com>,
 Xu Yilun <yilun.xu@...el.com>, Tom Rix <trix@...hat.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 linux-kernel@...r.kernel.org, linux-fpga@...r.kernel.org
Subject: Re: [RFC PATCH v4 1/1] fpga: add an owner and use it to take the
 low-level module's refcount



On 09/01/24 05:40, Xu Yilun wrote:
> On Mon, Jan 08, 2024 at 06:24:55PM +0100, Marco Pagani wrote:
>>
>>
>> On 2024-01-08 10:07, Xu Yilun wrote:
>>> On Sat, Jan 06, 2024 at 12:15:26AM +0100, Marco Pagani wrote:
>>>> Add a module owner field to the fpga_manager struct to take the
>>>> low-level control module refcount instead of assuming that the parent
>>>> device has a driver and using its owner pointer. The owner is now
>>>> passed as an additional argument at registration time. To this end,
>>>> the functions for registration have been modified to take an additional
>>>> owner parameter and renamed to avoid conflicts. The old function names
>>>> are now used for helper macros that automatically set the module that
>>>> registers the fpga manager as the owner. This ensures compatibility
>>>> with existing low-level control modules and reduces the chances of
>>>> registering a manager without setting the owner.
>>>>
>>>> To detect when the owner module pointer becomes stale, set the mops
>>>> pointer to null during fpga_mgr_unregister() and test it before taking
>>>> the module's refcount. Use a mutex to protect against a crash that can
>>>> happen if __fpga_mgr_get() gets suspended between testing the mops
>>>> pointer and taking the refcount while the low-level module is being
>>>> unloaded.
>>>>
>>>> Other changes: opportunistically move put_device() from __fpga_mgr_get()
>>>> to fpga_mgr_get() and of_fpga_mgr_get() to improve code clarity since
>>>> the device refcount in taken in these functions.
>>>>
>>>> Fixes: 654ba4cc0f3e ("fpga manager: ensure lifetime with of_fpga_mgr_get")
>>>> Suggested-by: Xu Yilun <yilun.xu@...el.com>
>>>> Signed-off-by: Marco Pagani <marpagan@...hat.com>
>>>> ---
>>>>  drivers/fpga/fpga-mgr.c       | 93 ++++++++++++++++++++++-------------
>>>>  include/linux/fpga/fpga-mgr.h | 80 +++++++++++++++++++++++++++---
>>>>  2 files changed, 134 insertions(+), 39 deletions(-)
>>>>
>>>> diff --git a/drivers/fpga/fpga-mgr.c b/drivers/fpga/fpga-mgr.c
>>>> index 06651389c592..d7bfbdfdf2fc 100644
>>>> --- a/drivers/fpga/fpga-mgr.c
>>>> +++ b/drivers/fpga/fpga-mgr.c
>>>> @@ -664,20 +664,20 @@ static struct attribute *fpga_mgr_attrs[] = {
>>>>  };
>>>>  ATTRIBUTE_GROUPS(fpga_mgr);
>>>>  
>>>> -static struct fpga_manager *__fpga_mgr_get(struct device *dev)
>>>> +static struct fpga_manager *__fpga_mgr_get(struct device *mgr_dev)
>>>>  {
>>>>  	struct fpga_manager *mgr;
>>>>  
>>>> -	mgr = to_fpga_manager(dev);
>>>> +	mgr = to_fpga_manager(mgr_dev);
>>>>  
>>>> -	if (!try_module_get(dev->parent->driver->owner))
>>>> -		goto err_dev;
>>>> +	mutex_lock(&mgr->mops_mutex);
>>>>  
>>>> -	return mgr;
>>>> +	if (!mgr->mops || !try_module_get(mgr->mops_owner))
>>>
>>> Why move the owner out of struct fpga_manager_ops? The owner within the
>>> ops struct makes more sense to me, it better illustrates what the mutex
>>> is protecting.
>>>
>>
>> I think having the owner in fpga_manager_ops made sense when it was
>> meant to be set while initializing the struct in the low-level module.
>> However, since the owner is now passed as an argument and set at
>> registration time, keeping it in the FPGA manager context seems more
>> straightforward to me.
> 
> That's OK. But then why not directly check mops_owner here?
>

We can do that, but it would impose a precondition since it would not
allow registering a manager with a NULL ops owner. In that case, I think
we need to make the precondition explicit and add a check in
fpga_mgr_register_*() that prevents registering a manager with a NULL ops
owner. Otherwise, the programmer could register a manager that cannot be
taken.
 
>>
>>  
>>>> +		mgr = ERR_PTR(-ENODEV);
>>>>  
>>>> -err_dev:
>>>> -	put_device(dev);
>>>> -	return ERR_PTR(-ENODEV);
>>>> +	mutex_unlock(&mgr->mops_mutex);
>>>> +
>>>> +	return mgr;
>>>>  }
>>>>  
>>>>  static int fpga_mgr_dev_match(struct device *dev, const void *data)
>>>> @@ -693,12 +693,18 @@ static int fpga_mgr_dev_match(struct device *dev, const void *data)
>>>>   */
>>>>  struct fpga_manager *fpga_mgr_get(struct device *dev)
>>>>  {
>>>> -	struct device *mgr_dev = class_find_device(&fpga_mgr_class, NULL, dev,
>>>> -						   fpga_mgr_dev_match);
>>>> +	struct fpga_manager *mgr;
>>>> +	struct device *mgr_dev;
>>>> +
>>>> +	mgr_dev = class_find_device(&fpga_mgr_class, NULL, dev, fpga_mgr_dev_match);
>>>>  	if (!mgr_dev)
>>>>  		return ERR_PTR(-ENODEV);
>>>>  
>>>> -	return __fpga_mgr_get(mgr_dev);
>>>> +	mgr = __fpga_mgr_get(mgr_dev);
>>>> +	if (IS_ERR(mgr))
>>>> +		put_device(mgr_dev);
>>>> +
>>>> +	return mgr;
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(fpga_mgr_get);
>>>>  
>>>> @@ -711,13 +717,18 @@ EXPORT_SYMBOL_GPL(fpga_mgr_get);
>>>>   */
>>>>  struct fpga_manager *of_fpga_mgr_get(struct device_node *node)
>>>>  {
>>>> -	struct device *dev;
>>>> +	struct fpga_manager *mgr;
>>>> +	struct device *mgr_dev;
>>>>  
>>>> -	dev = class_find_device_by_of_node(&fpga_mgr_class, node);
>>>> -	if (!dev)
>>>> +	mgr_dev = class_find_device_by_of_node(&fpga_mgr_class, node);
>>>> +	if (!mgr_dev)
>>>>  		return ERR_PTR(-ENODEV);
>>>>  
>>>> -	return __fpga_mgr_get(dev);
>>>> +	mgr = __fpga_mgr_get(mgr_dev);
>>>> +	if (IS_ERR(mgr))
>>>> +		put_device(mgr_dev);
>>>> +
>>>> +	return mgr;
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(of_fpga_mgr_get);
>>>>  
>>>> @@ -727,7 +738,7 @@ EXPORT_SYMBOL_GPL(of_fpga_mgr_get);
>>>>   */
>>>>  void fpga_mgr_put(struct fpga_manager *mgr)
>>>>  {
>>>> -	module_put(mgr->dev.parent->driver->owner);
>>>> +	module_put(mgr->mops_owner);
>>>>  	put_device(&mgr->dev);
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(fpga_mgr_put);
>>>> @@ -766,9 +777,10 @@ void fpga_mgr_unlock(struct fpga_manager *mgr)
>>>>  EXPORT_SYMBOL_GPL(fpga_mgr_unlock);
>>>>  
>>>>  /**
>>>> - * fpga_mgr_register_full - create and register an FPGA Manager device
>>>> + * __fpga_mgr_register_full - create and register an FPGA Manager device
>>>>   * @parent:	fpga manager device from pdev
>>>>   * @info:	parameters for fpga manager
>>>> + * @owner:	owner module containing the ops
>>>>   *
>>>>   * The caller of this function is responsible for calling fpga_mgr_unregister().
>>>>   * Using devm_fpga_mgr_register_full() instead is recommended.
>>>> @@ -776,7 +788,8 @@ EXPORT_SYMBOL_GPL(fpga_mgr_unlock);
>>>>   * Return: pointer to struct fpga_manager pointer or ERR_PTR()
>>>>   */
>>>>  struct fpga_manager *
>>>> -fpga_mgr_register_full(struct device *parent, const struct fpga_manager_info *info)
>>>> +__fpga_mgr_register_full(struct device *parent, const struct fpga_manager_info *info,
>>>> +			 struct module *owner)
>>>>  {
>>>>  	const struct fpga_manager_ops *mops = info->mops;
>>>>  	struct fpga_manager *mgr;
>>>> @@ -803,6 +816,9 @@ fpga_mgr_register_full(struct device *parent, const struct fpga_manager_info *in
>>>>  	}
>>>>  
>>>>  	mutex_init(&mgr->ref_mutex);
>>>> +	mutex_init(&mgr->mops_mutex);
>>>> +
>>>> +	mgr->mops_owner = owner;
>>>>  
>>>>  	mgr->name = info->name;
>>>>  	mgr->mops = info->mops;
>>>> @@ -841,14 +857,15 @@ fpga_mgr_register_full(struct device *parent, const struct fpga_manager_info *in
>>>>  
>>>>  	return ERR_PTR(ret);
>>>>  }
>>>> -EXPORT_SYMBOL_GPL(fpga_mgr_register_full);
>>>> +EXPORT_SYMBOL_GPL(__fpga_mgr_register_full);
>>>>  
>>>>  /**
>>>> - * fpga_mgr_register - create and register an FPGA Manager device
>>>> + * __fpga_mgr_register - create and register an FPGA Manager device
>>>>   * @parent:	fpga manager device from pdev
>>>>   * @name:	fpga manager name
>>>>   * @mops:	pointer to structure of fpga manager ops
>>>>   * @priv:	fpga manager private data
>>>> + * @owner:	owner module containing the ops
>>>>   *
>>>>   * The caller of this function is responsible for calling fpga_mgr_unregister().
>>>>   * Using devm_fpga_mgr_register() instead is recommended. This simple
>>>> @@ -859,8 +876,8 @@ EXPORT_SYMBOL_GPL(fpga_mgr_register_full);
>>>>   * Return: pointer to struct fpga_manager pointer or ERR_PTR()
>>>>   */
>>>>  struct fpga_manager *
>>>> -fpga_mgr_register(struct device *parent, const char *name,
>>>> -		  const struct fpga_manager_ops *mops, void *priv)
>>>> +__fpga_mgr_register(struct device *parent, const char *name,
>>>> +		    const struct fpga_manager_ops *mops, void *priv, struct module *owner)
>>>>  {
>>>>  	struct fpga_manager_info info = { 0 };
>>>>  
>>>> @@ -868,9 +885,9 @@ fpga_mgr_register(struct device *parent, const char *name,
>>>>  	info.mops = mops;
>>>>  	info.priv = priv;
>>>>  
>>>> -	return fpga_mgr_register_full(parent, &info);
>>>> +	return __fpga_mgr_register_full(parent, &info, owner);
>>>>  }
>>>> -EXPORT_SYMBOL_GPL(fpga_mgr_register);
>>>> +EXPORT_SYMBOL_GPL(__fpga_mgr_register);
>>>>  
>>>>  /**
>>>>   * fpga_mgr_unregister - unregister an FPGA manager
>>>> @@ -888,6 +905,12 @@ void fpga_mgr_unregister(struct fpga_manager *mgr)
>>>>  	 */
>>>>  	fpga_mgr_fpga_remove(mgr);
>>>>  
>>>> +	mutex_lock(&mgr->mops_mutex);
>>>> +
>>>> +	mgr->mops = NULL;
> 
> Same here, is it better set mgr->mops_owner = NULL?
> 
>>>> +
>>>> +	mutex_unlock(&mgr->mops_mutex);
>>>> +
>>>>  	device_unregister(&mgr->dev);
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(fpga_mgr_unregister);
>>>> @@ -900,9 +923,10 @@ static void devm_fpga_mgr_unregister(struct device *dev, void *res)
>>>>  }
>>>>  
>>>>  /**
>>>> - * devm_fpga_mgr_register_full - resource managed variant of fpga_mgr_register()
>>>> + * __devm_fpga_mgr_register_full - resource managed variant of fpga_mgr_register()
>>>>   * @parent:	fpga manager device from pdev
>>>>   * @info:	parameters for fpga manager
>>>> + * @owner:	owner module containing the ops
>>>>   *
>>>>   * Return:  fpga manager pointer on success, negative error code otherwise.
>>>>   *
>>>> @@ -910,7 +934,8 @@ static void devm_fpga_mgr_unregister(struct device *dev, void *res)
>>>>   * function will be called automatically when the managing device is detached.
>>>>   */
>>>>  struct fpga_manager *
>>>> -devm_fpga_mgr_register_full(struct device *parent, const struct fpga_manager_info *info)
>>>> +__devm_fpga_mgr_register_full(struct device *parent, const struct fpga_manager_info *info,
>>>> +			      struct module *owner)
>>>>  {
>>>>  	struct fpga_mgr_devres *dr;
>>>>  	struct fpga_manager *mgr;
>>>> @@ -919,7 +944,7 @@ devm_fpga_mgr_register_full(struct device *parent, const struct fpga_manager_inf
>>>>  	if (!dr)
>>>>  		return ERR_PTR(-ENOMEM);
>>>>  
>>>> -	mgr = fpga_mgr_register_full(parent, info);
>>>> +	mgr = __fpga_mgr_register_full(parent, info, owner);
>>>>  	if (IS_ERR(mgr)) {
>>>>  		devres_free(dr);
>>>>  		return mgr;
>>>> @@ -930,14 +955,15 @@ devm_fpga_mgr_register_full(struct device *parent, const struct fpga_manager_inf
>>>>  
>>>>  	return mgr;
>>>>  }
>>>> -EXPORT_SYMBOL_GPL(devm_fpga_mgr_register_full);
>>>> +EXPORT_SYMBOL_GPL(__devm_fpga_mgr_register_full);
>>>>  
>>>>  /**
>>>> - * devm_fpga_mgr_register - resource managed variant of fpga_mgr_register()
>>>> + * __devm_fpga_mgr_register - resource managed variant of fpga_mgr_register()
>>>>   * @parent:	fpga manager device from pdev
>>>>   * @name:	fpga manager name
>>>>   * @mops:	pointer to structure of fpga manager ops
>>>>   * @priv:	fpga manager private data
>>>> + * @owner:	owner module containing the ops
>>>>   *
>>>>   * Return:  fpga manager pointer on success, negative error code otherwise.
>>>>   *
>>>> @@ -946,8 +972,9 @@ EXPORT_SYMBOL_GPL(devm_fpga_mgr_register_full);
>>>>   * device is detached.
>>>>   */
>>>>  struct fpga_manager *
>>>> -devm_fpga_mgr_register(struct device *parent, const char *name,
>>>> -		       const struct fpga_manager_ops *mops, void *priv)
>>>> +__devm_fpga_mgr_register(struct device *parent, const char *name,
>>>> +			 const struct fpga_manager_ops *mops, void *priv,
>>>> +			 struct module *owner)
>>>>  {
>>>>  	struct fpga_manager_info info = { 0 };
>>>>  
>>>> @@ -955,9 +982,9 @@ devm_fpga_mgr_register(struct device *parent, const char *name,
>>>>  	info.mops = mops;
>>>>  	info.priv = priv;
>>>>  
>>>> -	return devm_fpga_mgr_register_full(parent, &info);
>>>> +	return __devm_fpga_mgr_register_full(parent, &info, owner);
>>>>  }
>>>> -EXPORT_SYMBOL_GPL(devm_fpga_mgr_register);
>>>> +EXPORT_SYMBOL_GPL(__devm_fpga_mgr_register);
>>>>  
>>>>  static void fpga_mgr_dev_release(struct device *dev)
>>>>  {
>>>> diff --git a/include/linux/fpga/fpga-mgr.h b/include/linux/fpga/fpga-mgr.h
>>>> index 54f63459efd6..967540311462 100644
>>>> --- a/include/linux/fpga/fpga-mgr.h
>>>> +++ b/include/linux/fpga/fpga-mgr.h
>>>> @@ -201,6 +201,8 @@ struct fpga_manager_ops {
>>>>   * @state: state of fpga manager
>>>>   * @compat_id: FPGA manager id for compatibility check.
>>>>   * @mops: pointer to struct of fpga manager ops
>>>> + * @mops_mutex: protects mops from low-level module removal
> 
> Same here, change the doc if needed.
> 
>>>> + * @mops_owner: module containing the mops
>>>>   * @priv: low level driver private date
>>>>   */
>>>>  struct fpga_manager {
>>>> @@ -210,6 +212,8 @@ struct fpga_manager {
>>>>  	enum fpga_mgr_states state;
>>>>  	struct fpga_compat_id *compat_id;
>>>>  	const struct fpga_manager_ops *mops;
>>>> +	struct mutex mops_mutex;
>>>> +	struct module *mops_owner;
>>>>  	void *priv;
>>>>  };
>>>>  
>>>> @@ -222,6 +226,7 @@ void fpga_image_info_free(struct fpga_image_info *info);
>>>>  int fpga_mgr_load(struct fpga_manager *mgr, struct fpga_image_info *info);
>>>>  
>>>>  int fpga_mgr_lock(struct fpga_manager *mgr);
>>>> +
>>>
>>> Why adding a line?
>>>
>>
>> I'll remove the line.
>>  
>>>>  void fpga_mgr_unlock(struct fpga_manager *mgr);
>>>>  
>>>>  struct fpga_manager *of_fpga_mgr_get(struct device_node *node);
>>>> @@ -230,18 +235,81 @@ struct fpga_manager *fpga_mgr_get(struct device *dev);
>>>>  
>>>>  void fpga_mgr_put(struct fpga_manager *mgr);
>>>>  
>>>> +/**
>>>> + * fpga_mgr_register_full - create and register an FPGA Manager device
>>>> + * @parent:	fpga manager device from pdev
>>>> + * @info:	parameters for fpga manager
>>>> + *
>>>> + * The caller of this function is responsible for calling fpga_mgr_unregister().
>>>> + * Using devm_fpga_mgr_register_full() instead is recommended.
>>>> + *
>>>> + * Return: pointer to struct fpga_manager pointer or ERR_PTR()
>>>> + */
>>>
>>> No need to duplicate the doc, just remove it.
>>> Same for the rest of code.
>>
>> Do you mean keeping the kernel-doc comments only for the __fpga_mgr_*
>> functions in fpga-mgr.c?
>>
>>>
>>>> +#define fpga_mgr_register_full(parent, info) \
>>>> +	__fpga_mgr_register_full(parent, info, THIS_MODULE)
>>>> +
>>>
>>> Delete the line, and ...
>>>
>>>>  struct fpga_manager *
>>>> -fpga_mgr_register_full(struct device *parent, const struct fpga_manager_info *info);
>>>> +__fpga_mgr_register_full(struct device *parent, const struct fpga_manager_info *info,
>>>> +			 struct module *owner);
>>>
>>> Add a line here, to make the related functions packed.
>>> Same for the rest of code.
>>
>> Do you prefer having the macro just above the function prototype? Like:
>>
>> #define devm_fpga_mgr_register(parent, name, mops, priv) \
>> 	__devm_fpga_mgr_register(parent, name, mops, priv, THIS_MODULE)
>> struct fpga_manager *
>> __devm_fpga_mgr_register(struct device *parent, const char *name,
>> 			 const struct fpga_manager_ops *mops, void *priv,
>> 			 struct module *owner);
> 
> Yes, that's it.

My only concern is that if we keep the kernel-doc comments only for the
__fpga_mgr_register* functions in fpga-mgr.c, we would not get the docs
for the helper macros that are the most likely to be used.


>> [...]


Thanks,
Marco

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