[PATCH v2] uio/uio_pci_generic: Introduce refcnt on open/release

[Yao Hongbo <yaohongbo@linux.alibaba.com>]

If two userspace programs both open the PCI UIO fd, when one
of the program exits uncleanly, the other will cause IO hang
due to bus-mastering disabled.

It's a common usage for spdk/dpdk to use UIO. So, introduce refcnt
to avoid such problems.

Fixes: 865a11f987ab("uio/uio_pci_generic: Disable bus-mastering on release")
Reported-by: Xiu Yang <yangxiu.yx@alibaba-inc.com>
Signed-off-by: Yao Hongbo <yaohongbo@linux.alibaba.com>
---
Changes for v2:
	Use refcount_t instead of atomic_t to catch overflow/underflows.
---
 drivers/uio/uio_pci_generic.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/uio/uio_pci_generic.c b/drivers/uio/uio_pci_generic.c
index e03f9b5..1a5e1fd 100644
--- a/drivers/uio/uio_pci_generic.c
+++ b/drivers/uio/uio_pci_generic.c
@@ -31,6 +31,7 @@
 struct uio_pci_generic_dev {
 	struct uio_info info;
 	struct pci_dev *pdev;
+	refcount_t refcnt;
 };
 
 static inline struct uio_pci_generic_dev *
@@ -39,6 +40,14 @@ struct uio_pci_generic_dev {
 	return container_of(info, struct uio_pci_generic_dev, info);
 }
 
+static int open(struct uio_info *info, struct inode *inode)
+{
+	struct uio_pci_generic_dev *gdev = to_uio_pci_generic_dev(info);
+
+	refcount_inc(&gdev->refcnt);
+	return 0;
+}
+
 static int release(struct uio_info *info, struct inode *inode)
 {
 	struct uio_pci_generic_dev *gdev = to_uio_pci_generic_dev(info);
@@ -51,7 +60,9 @@ static int release(struct uio_info *info, struct inode *inode)
 	 * Note that there's a non-zero chance doing this will wedge the device
 	 * at least until reset.
 	 */
-	pci_clear_master(gdev->pdev);
+	if (refcount_dec_and_test(&gdev->refcnt))
+		pci_clear_master(gdev->pdev);
+
 	return 0;
 }
 
@@ -92,8 +103,11 @@ static int probe(struct pci_dev *pdev,
 
 	gdev->info.name = "uio_pci_generic";
 	gdev->info.version = DRIVER_VERSION;
+	gdev->info.open = open;
 	gdev->info.release = release;
 	gdev->pdev = pdev;
+	refcount_set(&gdev->refcnt, 0);
+
 	if (pdev->irq && (pdev->irq != IRQ_NOTCONNECTED)) {
 		gdev->info.irq = pdev->irq;
 		gdev->info.irq_flags = IRQF_SHARED;
-- 
1.8.3.1