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SubjectRe: [PATCH bisected regression in 4.14] PCI: fix race while enabling upstream bridges concurrently
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On 15.09.2017 16:43, Srinath Mannam wrote:
> Hi Konstantin,
>
> On Fri, Sep 15, 2017 at 6:18 PM, Konstantin Khlebnikov
> <khlebnikov@yandex-team.ru> wrote:
>> In pci_enable_bridge() pci_enable_device() is called before calling
>> pci_set_master(), thus check pci_is_enabled() becomes true in the
>> middle of this sequence. As a result in pci_enable_device_flags()
>> concurrent enable of device on same bridge could think that this
>> bridge is completely enabled, but actually it's not yet.
>>
>> For me this race broke ethernet devices after booting kernel via
>> kexec, normal reboot was fine.
>>
>> This patch removes racy fast-path: pci_enable_bridge() will take
>> pci_bridge_mutex and do nothing if bridge is already enabled.
>>
>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
>> Fixes: 40f11adc7cd9 ("PCI: Avoid race while enabling upstream bridges")
>> ---
>> drivers/pci/pci.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>> index b0002daa50f3..ffbe11dbdd61 100644
>> --- a/drivers/pci/pci.c
>> +++ b/drivers/pci/pci.c
>> @@ -1394,7 +1394,7 @@ static int pci_enable_device_flags(struct pci_dev *dev, unsigned long flags)
>> return 0; /* already enabled */
>>
>> bridge = pci_upstream_bridge(dev);
>> - if (bridge && !pci_is_enabled(bridge))
>> + if (bridge)

> This patch causes deadlock because of nexted mutex lock.

Oh, yes.

I suppose not ascending to upstream bridge for bridges from pci_enable_device_flags should fix this.

Something like this:


--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -1393,9 +1393,11 @@ static int pci_enable_device_flags(struct pci_dev *dev, unsigned long flags)
if (atomic_inc_return(&dev->enable_cnt) > 1)
return 0; /* already enabled */

- bridge = pci_upstream_bridge(dev);
- if (bridge)
- pci_enable_bridge(bridge);
+ if (!pci_is_bridge(dev)) {
+ bridge = pci_upstream_bridge(dev);
+ if (bridge)
+ pci_enable_bridge(bridge);
+ }

/* only skip sriov related */
for (i = 0; i <= PCI_ROM_RESOURCE; i++)
> As per original code, Bridge enable function is called equal to number
> of child bridges it has.
> In the case endpoint is connected to RC through two bridges.
> bridge 2 is enabled(both device and bus master) first.
> While bridge1 enable, it calls device enable which calls device_enable_flags.
> set device enable flag
> check it has bridge (here yes because it has bridge2)
> calls bridge enable for bridge2. which is already enabled.
>
> So in my patch we introduced mutex to stop the race condition.
> By taking this mutex, we see dead lock in the second call for bridge
> enable (ex: bridge2)
> Here we stopped second time calling of bridge enable using "if (bridge
> && !pci_is_enabled(bridge))"
> In this case, there will not be such scenario where device enable and
> bus master is missed in bridge enable function.
> Because pci_is_enabled check in "if (bridge &&
> !pci_is_enabled(bridge))" will check for its bridge not itself.
> Stopping its bridge is not a problem because it is already enabled, as
> I explained above.
>
> Please explain your case where bus master could missed for bridge. It
> helps me to understand more about how various bridges enabled.

Pure race: seocnd deivice could call do_pci_enable_device() while
first still enabling bridge and haven't set bus master for it:


CPU1 CPU2

pci_enable_device_flags(dev1) pci_enable_device_flags(dev2)

pci_is_enabled(bridge)

atomic_read(&bridge->enable_cnt) -> 0

pci_upstream_bridge(bridge)

mutex_lock(&pci_bridge_mutex);

pci_enable_device(bridge)

atomic_inc_return(&bridge->enable_cnt) 0 -> 1

pci_is_enabled(bridge)

atomic_read(&bridge->enable_cnt) -> 1

do_pci_enable_device(dev2) -> fail,

pci_set_master(bridge);

mutex_unlock(&pci_bridge_mutex);

do_pci_enable_device(dev1)


>
>> pci_enable_bridge(bridge);
>>
>> /* only skip sriov related */
>>
> Regards,
> Srinath.
>

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