Re: [RFC PATCH] fpga: remove module reference counting from core components

[Xu Yilun]

On Fri, Nov 03, 2023 at 09:31:02PM +0100, Marco Pagani wrote:
> 
> 
> On 2023-10-30 09:32, Xu Yilun wrote:
> > On Fri, Oct 27, 2023 at 05:29:27PM +0200, Marco Pagani wrote:
> >> Remove unnecessary module reference counting from the core components
> >> of the subsystem. Low-level driver modules cannot be removed before
> >> core modules since they use their exported symbols.
> > 
> > Could you help show the code for this conclusion?
> > 
> > This is different from what I remember, a module cannot be removed when
> > its exported symbols are being used by other modules. IOW, the core
> > modules cannot be removed when there exist related low-level driver
> > modules. But the low-level driver modules could be removed freely
> > without other protecting mechanism.
> >
> 
> My understanding was that we wanted to remove module reference counting
> from the fpga core and ease it from the responsibility of preventing
> low-level driver modules from being unloaded. 

FPGA core needs to prevent low-level driver module unloading sometimes,
e.g. when region reprograming is in progress. That's why we get fpga
region driver modules & bridge modules in fpga_region_program_fpga().

But we try best to get them only necessary. Blindly geting them all the
time results in no way to unload all modules (core & low level modules).

> 
> If we want to keep reference counting in the fpga core, we could add a
> struct module *owner field in the struct fpga_manager_ops (and others
> core *_ops) so that the low-level driver can set it to THIS_MODULE.
> In this way, we can later use it in fpga_mgr_register() to bump up the

Yes, we should pass the module owner in fpga_mgr_register(), but could
not bump up its refcount at once.

> refcount of the low-level driver module by calling
> try_module_get(mgr->mops->owner) directly when it registers the manager.
> Finally, fpga_mgr_unregister() would call module_put(mgr->mops->owner)
> to allow unloading the low-level driver module.

As mentioned above, that makes problem. Most of the low level driver
modules call fpga_mgr_unregister() on module_exit(), but bumping up
their module refcount prevents module_exit() been executed. That came
out to be a dead lock.

> 
> In this way, it would no longer be necessary to call try_module_get()
> in fpga_mrg_get() since we could use a kref (included in the struct
> fpga_manager) to do refcounting for the in-kernel API users. Only when
> the kref reaches zero fpga_mgr_unregister() would succeed and put the
> low-level driver module.
> 
> I think this approach would be safer since it would avoid the crash
> that can currently happen if the low-level driver module is removed
> right when executing try_module_get() in fpga_mrg_get(). The possible
> caveat is that it would be required to call fpga_mgr_unregister()
> before being able to remove the low-level driver module.
> 
> >>
> >> For more context, refer to this thread:
> >> https://lore.kernel.org/linux-fpga/ZS6hhlvjUcqyv8zL@yilunxu-OptiPlex-7050
> >>
> >> Other changes:
> >>
> >> In  __fpga_bridge_get(): do a (missing ?) get_device() and bind the
> > 
> > I think get_device() is in (of)_fpga_bridge_get() -> class_find_device()
> > and put_device() correspond to it.
> >
> 
> You are right. I missed that one.
>  
> > But the code style here is rather misleading, the put_device() should be
> > moved out to (of)_fpga_bridge_get().
> >
> 
> Right, I'll improve the (of)_fpga_bridge_get() style for the next version.
> 
> >> image to the bridge only after the mutex has been acquired.
> > 
> > This is good to me.
> > 
> >>
> >> In __fpga_mgr_get(): do a get_device(). Currently, get_device() is
> >> called when allocating an image in fpga_image_info_alloc().
> >> However, since there are still two (of_)fpga_mgr_get() functions
> >> exposed by the core, I think they should behave as expected.
> > 
> > Same as fpga bridge.
> > 
> >>
> >> In fpga_region_get() / fpga_region_put(): call get_device() before
> >> acquiring the mutex and put_device() after having released the mutex
> >> to avoid races.
> > 
> > Could you help elaborate more about the race?
> > 
> 
> I accidentally misused the word race. My concern was that memory might
> be released after the last put_device(), causing mutex_unlock() to be
> called on a mutex that does not exist anymore. It should not happen
> for the moment since the region does not use devres, but I think it
> still makes the code more brittle.

It makes sense.

But I dislike the mutex itself. The purpose is to exclusively grab the
device, but a mutex is too much heavy for this. The lock/unlock of mutex
scattered in different functions is also an uncommon style. Maybe some
atomic count should be enough.

Thanks,
Yilun


> 
> > Thanks,
> > Yilun
> > 
> >>
> >> Fixes: 654ba4cc0f3e ("fpga manager: ensure lifetime with of_fpga_mgr_get")
> >> Signed-off-by: Marco Pagani <marpagan@xxxxxxxxxx>
> >> ---
> >>  drivers/fpga/fpga-bridge.c | 24 +++++++-----------------
> >>  drivers/fpga/fpga-mgr.c    |  8 +-------
> >>  drivers/fpga/fpga-region.c | 14 ++++----------
> >>  3 files changed, 12 insertions(+), 34 deletions(-)
> >>
> >> diff --git a/drivers/fpga/fpga-bridge.c b/drivers/fpga/fpga-bridge.c
> >> index a024be2b84e2..3bcc9c9849c5 100644
> >> --- a/drivers/fpga/fpga-bridge.c
> >> +++ b/drivers/fpga/fpga-bridge.c
> >> @@ -58,30 +58,21 @@ EXPORT_SYMBOL_GPL(fpga_bridge_disable);
> >>  static struct fpga_bridge *__fpga_bridge_get(struct device *dev,
> >>  					     struct fpga_image_info *info)
> >>  {
> >> -	struct fpga_bridge *bridge;
> >> -	int ret = -ENODEV;
> >> -
> >> -	bridge = to_fpga_bridge(dev);
> >> +	struct fpga_bridge *bridge = to_fpga_bridge(dev);
> >>  
> >> -	bridge->info = info;
> >> +	get_device(dev);
> >>  
> >>  	if (!mutex_trylock(&bridge->mutex)) {
> >> -		ret = -EBUSY;
> >> -		goto err_dev;
> >> +		dev_dbg(dev, "%s: FPGA Bridge already in use\n", __func__);
> >> +		put_device(dev);
> >> +		return ERR_PTR(-EBUSY);
> >>  	}
> >>  
> >> -	if (!try_module_get(dev->parent->driver->owner))
> >> -		goto err_ll_mod;
> >> +	bridge->info = info;
> >>  
> >>  	dev_dbg(&bridge->dev, "get\n");
> >>  
> >>  	return bridge;
> >> -
> >> -err_ll_mod:
> >> -	mutex_unlock(&bridge->mutex);
> >> -err_dev:
> >> -	put_device(dev);
> >> -	return ERR_PTR(ret);
> >>  }
> >>  
> >>  /**
> >> @@ -93,7 +84,7 @@ static struct fpga_bridge *__fpga_bridge_get(struct device *dev,
> >>   * Return:
> >>   * * fpga_bridge struct pointer if successful.
> >>   * * -EBUSY if someone already has a reference to the bridge.
> >> - * * -ENODEV if @np is not an FPGA Bridge or can't take parent driver refcount.
> >> + * * -ENODEV if @np is not an FPGA Bridge.
> >>   */
> >>  struct fpga_bridge *of_fpga_bridge_get(struct device_node *np,
> >>  				       struct fpga_image_info *info)
> >> @@ -146,7 +137,6 @@ void fpga_bridge_put(struct fpga_bridge *bridge)
> >>  	dev_dbg(&bridge->dev, "put\n");
> >>  
> >>  	bridge->info = NULL;
> >> -	module_put(bridge->dev.parent->driver->owner);
> >>  	mutex_unlock(&bridge->mutex);
> >>  	put_device(&bridge->dev);
> >>  }
> >> diff --git a/drivers/fpga/fpga-mgr.c b/drivers/fpga/fpga-mgr.c
> >> index 06651389c592..6c355eafd18f 100644
> >> --- a/drivers/fpga/fpga-mgr.c
> >> +++ b/drivers/fpga/fpga-mgr.c
> >> @@ -670,14 +670,9 @@ static struct fpga_manager *__fpga_mgr_get(struct device *dev)
> >>  
> >>  	mgr = to_fpga_manager(dev);
> >>  
> >> -	if (!try_module_get(dev->parent->driver->owner))
> >> -		goto err_dev;
> >> +	get_device(&mgr->dev);
> >>  
> >>  	return mgr;
> >> -
> >> -err_dev:
> >> -	put_device(dev);
> >> -	return ERR_PTR(-ENODEV);
> >>  }
> >>  
> >>  static int fpga_mgr_dev_match(struct device *dev, const void *data)
> >> @@ -727,7 +722,6 @@ EXPORT_SYMBOL_GPL(of_fpga_mgr_get);
> >>   */
> >>  void fpga_mgr_put(struct fpga_manager *mgr)
> >>  {
> >> -	module_put(mgr->dev.parent->driver->owner);
> >>  	put_device(&mgr->dev);
> >>  }
> >>  EXPORT_SYMBOL_GPL(fpga_mgr_put);
> >> diff --git a/drivers/fpga/fpga-region.c b/drivers/fpga/fpga-region.c
> >> index b364a929425c..c299956cafdc 100644
> >> --- a/drivers/fpga/fpga-region.c
> >> +++ b/drivers/fpga/fpga-region.c
> >> @@ -41,22 +41,17 @@ EXPORT_SYMBOL_GPL(fpga_region_class_find);
> >>   * Return:
> >>   * * fpga_region struct if successful.
> >>   * * -EBUSY if someone already has a reference to the region.
> >> - * * -ENODEV if can't take parent driver module refcount.
> >>   */
> >>  static struct fpga_region *fpga_region_get(struct fpga_region *region)
> >>  {
> >>  	struct device *dev = &region->dev;
> >>  
> >> +	get_device(dev);
> >> +
> >>  	if (!mutex_trylock(&region->mutex)) {
> >>  		dev_dbg(dev, "%s: FPGA Region already in use\n", __func__);
> >> -		return ERR_PTR(-EBUSY);
> >> -	}
> >> -
> >> -	get_device(dev);
> >> -	if (!try_module_get(dev->parent->driver->owner)) {
> >>  		put_device(dev);
> >> -		mutex_unlock(&region->mutex);
> >> -		return ERR_PTR(-ENODEV);
> >> +		return ERR_PTR(-EBUSY);
> >>  	}
> >>  
> >>  	dev_dbg(dev, "get\n");
> >> @@ -75,9 +70,8 @@ static void fpga_region_put(struct fpga_region *region)
> >>  
> >>  	dev_dbg(dev, "put\n");
> >>  
> >> -	module_put(dev->parent->driver->owner);
> >> -	put_device(dev);
> >>  	mutex_unlock(&region->mutex);
> >> +	put_device(dev);
> >>  }
> >>  
> >>  /**
> >> -- 
> >> 2.41.0
> >>
> > 
>