Hi Srinath,
Thanks for chasing this down! It must have been a real hassle to find
this.
On Mon, May 08, 2017 at 08:39:50PM +0530, Srinath Mannam wrote:
> 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
> 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. Done Atomic increment dev->enable_cnt in function
> pci_enable_device_flag fail return 0 from there
> 2. Gone into pci_set_master
> 3. Completed pci_read_config_word(dev, PCI_COMMAND, &old_cmd)
> 4. 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.
Most places that write PCI_COMMAND do this sort of read/modify/write
thing. But in most of those places we only touch one device, and it's
the device we own, e.g., the PCI core enumerating it or a driver.
I don't think we really have a concurrency problem with those.
But in this case, where a driver called pci_enable_device() for device
A, and we're touching an upstream bridge B, we *do* have an issue.
Since it's a concurrency issue, the obvious solution is a mutex. I'm
sure this patch solves it, but it's not obvious to me how it works and
I'd be afraid of breaking it in the future.
Would it work to add a mutex in the struct pci_host_bridge, then hold
it while we enable upstream bridges, e.g., something like the
following?
bridge = pci_upstream_bridge(dev);
if (bridge) {
struct mutex = &pci_find_host_bridge(dev)->mutex;
mutex_lock(mutex);
pci_enable_bridge(bridge);
mutex_unlock(mutex);
}
Bjorn
> Signed-off-by: Srinath Mannam <srinath.mannam@xxxxxxxxxxxx>
>
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 7904d02..6c5744e 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -1345,21 +1345,39 @@ static void pci_enable_bridge(struct pci_dev *dev)
> {
> struct pci_dev *bridge;
> int retval;
> + int err;
> + int i;
> + unsigned int bars = 0;
> + unsigned long flags = IORESOURCE_MEM | IORESOURCE_IO;
>
> bridge = pci_upstream_bridge(dev);
> if (bridge)
> pci_enable_bridge(bridge);
>
> - if (pci_is_enabled(dev)) {
> - if (!dev->is_busmaster)
> - pci_set_master(dev);
> + if (dev->pm_cap) {
> + u16 pmcsr;
> +
> + pci_read_config_word(dev, dev->pm_cap + PCI_PM_CTRL, &pmcsr);
> + dev->current_state = (pmcsr & PCI_PM_CTRL_STATE_MASK);
> + }
> +
> + if (atomic_inc_return(&dev->enable_cnt) > 1)
> + return; /* already enabled */
> +
> + /* only skip sriov related */
> + for (i = 0; i <= PCI_ROM_RESOURCE; i++)
> + if (dev->resource[i].flags & flags)
> + bars |= (1 << i);
> + for (i = PCI_BRIDGE_RESOURCES; i < DEVICE_COUNT_RESOURCE; i++)
> + if (dev->resource[i].flags & flags)
> + bars |= (1 << i);
> +
> + err = do_pci_enable_device(dev, bars);
> + if (err < 0) {
> + atomic_dec(&dev->enable_cnt);
> return;
> }
>
> - retval = pci_enable_device(dev);
> - if (retval)
> - dev_err(&dev->dev, "Error enabling bridge (%d), continuing\n",
> - retval);
> pci_set_master(dev);
> }
>
> --
> 2.7.4
>
