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From:   Marco Pagani <marpagan@redhat.com>
To:     Moritz Fischer <mdf@kernel.org>, Wu Hao <hao.wu@intel.com>,
        Xu Yilun <yilun.xu@intel.com>, Tom Rix <trix@redhat.com>,
        Alan Tull <atull@opensource.altera.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Marco Pagani <marpagan@redhat.com>, linux-fpga@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: [RFC PATCH] fpga: remove module reference counting from core components
Date:   Fri, 27 Oct 2023 17:29:27 +0200
Message-ID: <20231027152928.184012-1-marpagan@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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.

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
image to the bridge only after the mutex has been acquired.

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.

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.

Fixes: 654ba4cc0f3e ("fpga manager: ensure lifetime with of_fpga_mgr_get")
Signed-off-by: Marco Pagani <marpagan@redhat.com>
---
 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