From: Weifeng Su <suweifeng1@huawei.com>

The /dev/uioX device has concurrent operations in a few scenarios.

For example, when a process using the device exits abnormally,
the management program starts the same process to operate the device.
When the process exits and closes the /dev/uioX device,
the master bit of the device is cleared. In this case, if the
new process is issuing commands, I/Os are suspended and cannot be
automatically recovered.

Therefore, reference counting is added to clear the master bit
only when the last process exits.

Signed-off-by: Weifeng Su <suweifeng1@huawei.com>
---
The difference between the first patch and the first patch is that 
the reference counting operation is performed using the atomic semantics, 
just like other drivers under UIO:
cdfa835c6e5e87d145f("uio_hv_generic: defer opening vmbus until first use").

 drivers/uio/uio_pci_generic.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/uio/uio_pci_generic.c b/drivers/uio/uio_pci_generic.c
index e03f9b532..df436e3d9 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;
+	atomic_t	refcnt;
 };
 
 static inline struct uio_pci_generic_dev *
@@ -39,10 +40,19 @@ to_uio_pci_generic_dev(struct uio_info *info)
 	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);
+
+	atomic_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);
 
+
 	/*
 	 * This driver is insecure when used with devices doing DMA, but some
 	 * people (mis)use it with such devices.
@@ -51,7 +61,8 @@ 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 (atomic_dec_and_test(&gdev->refcnt))
+		pci_clear_master(gdev->pdev);
 	return 0;
 }
 
@@ -93,7 +104,9 @@ static int probe(struct pci_dev *pdev,
 	gdev->info.name = "uio_pci_generic";
 	gdev->info.version = DRIVER_VERSION;
 	gdev->info.release = release;
+	gdev->info.open = open;
 	gdev->pdev = pdev;
+	atomic_set(&gdev->refcnt, 0);
 	if (pdev->irq && (pdev->irq != IRQ_NOTCONNECTED)) {
 		gdev->info.irq = pdev->irq;
 		gdev->info.irq_flags = IRQF_SHARED;
-- 
2.33.0