[RFC PATCH v2] pci: Concurrency issue in NVMe Init through PCIe switch

[Srinath Mannam]

We found a concurrency issue in NVMe Init when we initialize
multiple NVMe connected over PCIe switch.

Setup details:
 - SMP system with 8 ARMv8 cores running Linux kernel(4.11).
 - Two NVMe cards are connected to PCIe RC through bridge as shown
   in the below figure.

                   [RC]
                    |
                 [BRIDGE]
                    |
               -----------
              |           |
            [NVMe]      [NVMe]

Issue description:
After PCIe enumeration completed NVMe driver probe function called
for both the devices from two CPUS simultaneously.
>From nvme_probe, pci_enable_device_mem called for both the EPs. This
function called pci_enable_bridge enable recursively until RC.

Inside pci_enable_bridge function, at two places concurrency issue is
observed.

Place 1:
  CPU 0:
    1. Done Atomic increment dev->enable_cnt
       in pci_enable_device_flags
    2. Inside pci_enable_resources
    3. Completed pci_read_config_word(dev, PCI_COMMAND, &cmd)
    4. Ready to set PCI_COMMAND_MEMORY (0x2) in
       pci_write_config_word(dev, PCI_COMMAND, cmd)
  CPU 1:
    1. Check pci_is_enabled in function pci_enable_bridge
       and it is true
    2. Check (!dev->is_busmaster) also true
    3. Gone into pci_set_master
    4. Completed pci_read_config_word(dev, PCI_COMMAND, &old_cmd)
    5. Ready to set PCI_COMMAND_MASTER (0x4) in
       pci_write_config_word(dev, PCI_COMMAND, cmd)

By the time of last point for both the CPUs are read value 0 and
ready to write 2 and 4.
After last point final value in PCI_COMMAND register is 4 instead of 6.

Place 2:
  CPU 0:
    1. Done Atomic increment dev->enable_cnt in
       pci_enable_device_flags

Signed-off-by: Srinath Mannam <srinath.mannam@xxxxxxxxxxxx>
---
Changes since v1:
 - Used mutex to syncronize pci_enable_bridge

 drivers/pci/pci.c   | 4 ++++
 drivers/pci/probe.c | 1 +
 include/linux/pci.h | 1 +
 3 files changed, 6 insertions(+)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index b01bd5b..5bff3e7 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -1347,7 +1347,9 @@ static void pci_enable_bridge(struct pci_dev *dev)
 {
 	struct pci_dev *bridge;
 	int retval;
+	struct mutex *lock = &dev->bridge_lock;
 
+	mutex_lock(lock);
 	bridge = pci_upstream_bridge(dev);
 	if (bridge)
 		pci_enable_bridge(bridge);
@@ -1355,6 +1357,7 @@ static void pci_enable_bridge(struct pci_dev *dev)
 	if (pci_is_enabled(dev)) {
 		if (!dev->is_busmaster)
 			pci_set_master(dev);
+		mutex_unlock(lock);
 		return;
 	}
 
@@ -1363,6 +1366,7 @@ static void pci_enable_bridge(struct pci_dev *dev)
 		dev_err(&dev->dev, "Error enabling bridge (%d), continuing\n",
 			retval);
 	pci_set_master(dev);
+	mutex_unlock(lock);
 }
 
 static int pci_enable_device_flags(struct pci_dev *dev, unsigned long flags)
diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 19c8950..1c25d1c 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -880,6 +880,7 @@ static struct pci_bus *pci_alloc_child_bus(struct pci_bus *parent,
 	child->dev.parent = child->bridge;
 	pci_set_bus_of_node(child);
 	pci_set_bus_speed(child);
+	mutex_init(&bridge->bridge_lock);
 
 	/* Set up default resource pointers and names.. */
 	for (i = 0; i < PCI_BRIDGE_RESOURCE_NUM; i++) {
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 33c2b0b..7e88f41 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -266,6 +266,7 @@ struct pci_dev {
 	void		*sysdata;	/* hook for sys-specific extension */
 	struct proc_dir_entry *procent;	/* device entry in /proc/bus/pci */
 	struct pci_slot	*slot;		/* Physical slot this device is in */
+	struct mutex bridge_lock;
 
 	unsigned int	devfn;		/* encoded device & function index */
 	unsigned short	vendor;
-- 
2.7.4