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SubjectRe: [RFC PATCH v3] pci: Concurrency issue during pci enable bridge
Hi Bjorn,

Thank you for the feedback.

My comments are in lined.

On Wed, Aug 16, 2017 at 7:13 PM, Bjorn Helgaas <helgaas@kernel.org> 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@broadcom.com>
>> ---
>> 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)

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