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

[Ray Jui <ray.jui@broadcom.com>]

Hi Srinath and Scott,

On 5/30/17 8:44 AM, Scott Branden wrote:
> Hi Srinath,
> 
> 
> On 17-05-30 02:08 AM, 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
>>
>> Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
>> ---
>> 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);
>>   }
> Looking at above function I think it should be restructured so that
> mute_unlock only needs to be called in one place.
> How about below to make things more clear?
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 563901c..82c232e 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -1347,22 +1347,29 @@ static void pci_enable_bridge(struct pci_dev *dev)
>  {
>         struct pci_dev *bridge;
>         int retval;
> +       struct mutex *lock = &dev->bridge_lock;

First of all, using a proper lock for this was proposed during the
internal review. The idea was dismissed with concern that a dead lock
can happen since the call to pci_enable_bridge is recursive.

I'm glad that we are now still using the lock to properly fix the
problem by realizing that the lock is specific to the bridge itself and
the recursive call to upstream bridge does not cause a deadlock since a
different lock is used.

> +
> +       /*
> +        * Add comment here explaining what needs concurrency protection
> +        */
> +       mutex_lock(lock);
> 
>         bridge = pci_upstream_bridge(dev);
>         if (bridge)
>                 pci_enable_bridge(bridge);
> 
> -       if (pci_is_enabled(dev)) {
> -               if (!dev->is_busmaster)
> -                       pci_set_master(dev);
> -               return;
> +       if (!pci_is_enabled(dev)) {
> +               retval = pci_enable_device(dev);
> +               if (retval)
> +                       dev_err(&dev->dev,
> +                               "Error enabling bridge (%d), continuing\n",
> +                               retval);
>         }
> 
> -       retval = pci_enable_device(dev);
> -       if (retval)
> -               dev_err(&dev->dev, "Error enabling bridge (%d),
> continuing\n",
> -                       retval);
> -       pci_set_master(dev);
> +       if (!dev->is_busmaster)
> +               pci_set_master(dev);
> +
> +       mutex_unlock(lock);
>  }
> 

I really don't see how the above logic is cleaner than the change from
Srinath and personally I found this is way harder to read. And we are
doing all of this just to keep the mutex_unlock in one place.

If that is desired, a label at the end of the function like this will do:

out:
    mutex_unlock(lock);

And error case in the middle of the function can bail out and jump to
the label. Note I do not invent this. This is commonly done in a lot of
drivers in the kernel for cleaner error handling code.

>>     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;
> Regards,
>  Scott