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

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?

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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