Re: [RFC PATCH v3] pci: Concurrency issue during pci enable bridge

[Srinath Mannam]

Hi Bjorn,

Thank you for the feedback.

My comments are in lined.

On Wed, Aug 16, 2017 at 7:13 PM, Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> On Fri, Aug 04, 2017 at 08:27:28PM +0530, Srinath Mannam wrote:
>> Concurrency issue is observed during pci enable bridge called
>> for multiple pci devices initialization in SMP system.
>>
>> Setup details:
>>  - SMP system with 8 ARMv8 cores running Linux kernel(4.11).
>>  - Two EPs are connected to PCIe RC through bridge as shown
>>    in the below figure.
>>
>>                    [RC]
>>                     |
>>                  [BRIDGE]
>>                     |
>>                -----------
>>               |           |
>>              [EP]        [EP]
>>
>> Issue description:
>> After PCIe enumeration completed EP driver probe function called
>> for both the devices from two CPUS simultaneously.
>> From EP probe function, pci_enable_device_mem called for both the EPs.
>> This function called pci_enable_bridge enable for all the bridges
>> recursively in the path of EP to 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>
>> ---
>>  drivers/pci/pci.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>> index af0cc34..12721df 100644
>> --- a/drivers/pci/pci.c
>> +++ b/drivers/pci/pci.c
>> @@ -52,6 +52,7 @@ static void pci_pme_list_scan(struct work_struct *work);
>>  static LIST_HEAD(pci_pme_list);
>>  static DEFINE_MUTEX(pci_pme_list_mutex);
>>  static DECLARE_DELAYED_WORK(pci_pme_work, pci_pme_list_scan);
>> +static DEFINE_MUTEX(pci_bridge_mutex);
>>
>>  struct pci_pme_device {
>>       struct list_head list;
>> @@ -1348,10 +1349,11 @@ static void pci_enable_bridge(struct pci_dev *dev)
>>       if (bridge)
>>               pci_enable_bridge(bridge);
>>
>> +     mutex_lock(&pci_bridge_mutex);
>>       if (pci_is_enabled(dev)) {
>>               if (!dev->is_busmaster)
>>                       pci_set_master(dev);
>> -             return;
>> +             goto end;
>>       }
>>
>>       retval = pci_enable_device(dev);
>> @@ -1359,6 +1361,8 @@ static void pci_enable_bridge(struct pci_dev *dev)
>>               dev_err(&dev->dev, "Error enabling bridge (%d), continuing\n",
>>                       retval);
>>       pci_set_master(dev);
>> +end:
>> +     mutex_unlock(&pci_bridge_mutex);
>
> I think this will deadlock because we're holding pci_bridge_mutex
> while we call pci_enable_device(), which may recursively call
> pci_enable_bridge(), which would try to acquire pci_bridge_mutex
> again.  My original suggestion of a mutex in the host bridge would
> have the same problem.

This extra check "if (bridge && !pci_is_enabled(bridge))" will resolve
the deadlock in the present patch.
>
> We talked about using device_lock() earlier.  You found some problems
> with that, and I'd like to understand them better.  You said:
>
>> But the pci_enable_bridge is called in the context of the driver
>> probe function, we will have nexted lock problem.
>
> The driver core does hold device_lock() while calling the driver probe
> function, in this path:
>
>   device_initial_probe
>     __device_attach
>       device_lock(dev)                # <-- lock
>       __device_attach_driver
>         ...
>           pci_device_probe
>             ...
>               ->probe                 # driver probe function
>       device_unlock(dev)              # <-- unlock
>
> I didn't see your patch using device_lock(), but what I had in mind
> was something like the patch below, where pci_enable_bridge() acquires
> the device_lock() of the bridge.
>
> For the sake of argument, assume a hierarchy:
>
>   bridge A -> bridge B -> endpoint C
>
> Here's what I think will happen:
>
>   device_lock(C)                         # driver core
>     ...
>       ->probe(C)                         # driver probe function
>         pci_enable_device_flags(C)
>           pci_enable_bridge(B)           # enable C's upstream bridge
>             device_lock(B)
>             pci_enable_bridge(A)         # enable B's upstream bridge
>               device_lock(A)             # A has no upstream bridge
>               pci_enable_device(A)
>                 do_pci_enable_device(A)  # update A PCI_COMMAND
>               pci_set_master(A)          # update A PCI_COMMAND
>               device_unlock(A)
>             pci_enable_device(B)         # update B PCI_COMMAND
>             pci_set_master(B)            # update B PCI_COMMAND
>             device_unlock(B)
>           do_pci_enable_device(C)        # update C PCI_COMMAND
>   device_unlock(C)
>
> I don't see a nested lock problem here.  What am I missing?
>From the probe call device_lock will taken to that endpoint and also
for the bus. In this order

pci register driver(C)              #(driver_register())
device_lock(B);                       # lock for parent (__driver_attach())
device_lock(C)                       # lock for endpoint (__driver_attach())
driver probe(C)
pci_enable_bridge()
device_lock(B);  # here we see the deadlock.because of parent device lock

>
> Bjorn
>
>
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index e8e40dea2842..38154ba628a9 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -1344,6 +1344,7 @@ static void pci_enable_bridge(struct pci_dev *dev)
>         struct pci_dev *bridge;
>         int retval;
>
> +       device_lock(&dev->dev);
>         bridge = pci_upstream_bridge(dev);
>         if (bridge)
>                 pci_enable_bridge(bridge);
> @@ -1351,7 +1352,7 @@ static void pci_enable_bridge(struct pci_dev *dev)
>         if (pci_is_enabled(dev)) {
>                 if (!dev->is_busmaster)
>                         pci_set_master(dev);
> -               return;
> +               goto out;
>         }
>
>         retval = pci_enable_device(dev);
> @@ -1359,6 +1360,9 @@ static void pci_enable_bridge(struct pci_dev *dev)
>                 dev_err(&dev->dev, "Error enabling bridge (%d), continuing\n",
>                         retval);
>         pci_set_master(dev);
> +
> +out:
> +       device_unlock(&dev->dev);
>  }
>
>  static int pci_enable_device_flags(struct pci_dev *dev, unsigned long flags)