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Subject: Re: [PATCH v6 2/2] PCI/ERR: Split the fatal and non-fatal error
 recovery handling
To:     Ethan Zhao <xerces.zhao@gmail.com>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.nkuppuswamy@gmail.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Sinan Kaya <okaya@kernel.org>,
        linux-pci <linux-pci@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ashok Raj <ashok.raj@intel.com>
References: <546d346644654915877365b19ea534378db0894d.1602663397.git.sathyanarayanan.kuppuswamy@linux.intel.com>
 <d97541df3b44822e0d085ffa058e9e7c0ba05214.1602663397.git.sathyanarayanan.kuppuswamy@linux.intel.com>
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 <17e142b8-b19a-0ec7-833b-7a4ac2e76d0d@linux.intel.com>
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 <b84ae5fd-d1db-9378-7e2e-937b660d2e9a@linux.intel.com>
 <CAKF3qh107RGykkHCXhCzfq+A6COQWri8svsUfukF9PySHW-qQA@mail.gmail.com>
From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
Message-ID: <eb94df0b-4cb9-eb49-576a-87ac43fcfdfb@linux.intel.com>
Date:   Wed, 14 Oct 2020 22:53:36 -0700
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On 10/14/20 10:05 PM, Ethan Zhao wrote:
> On Thu, Oct 15, 2020 at 11:04 AM Kuppuswamy, Sathyanarayanan
> <sathyanarayanan.kuppuswamy@linux.intel.com> wrote:
>>
>>
>>
>> On 10/14/20 6:58 PM, Ethan Zhao wrote:
>>> On Thu, Oct 15, 2020 at 1:06 AM Kuppuswamy, Sathyanarayanan
>>> <sathyanarayanan.kuppuswamy@linux.intel.com> wrote:
>>>>
>>>>
>>>>
>>>> On 10/14/20 8:07 AM, Ethan Zhao wrote:
>>>>> On Wed, Oct 14, 2020 at 5:00 PM Kuppuswamy Sathyanarayanan
>>>>> <sathyanarayanan.nkuppuswamy@gmail.com> wrote:
>>>>>>
>>>>>> Commit bdb5ac85777d ("PCI/ERR: Handle fatal error recovery")
>>>>>> merged fatal and non-fatal error recovery paths, and also made
>>>>>> recovery code depend on hotplug handler for "remove affected
>>>>>> device + rescan" support. But this change also complicated the
>>>>>> error recovery path and which in turn led to the following
>>>>>> issues.
>>>>>>
>>>>>> 1. We depend on hotplug handler for removing the affected
>>>>>> devices/drivers on DLLSC LINK down event (on DPC event
>>>>>> trigger) and DPC handler for handling the error recovery. Since
>>>>>> both handlers operate on same set of affected devices, it leads
>>>>>> to race condition, which in turn leads to  NULL pointer
>>>>>> exceptions or error recovery failures.You can find more details
>>>>>> about this issue in following link.
>>>>>>
>>>>>> https://lore.kernel.org/linux-pci/20201007113158.48933-1-haifeng.zhao@intel.com/T/#t
>>>>>>
>>>>>> 2. For non-hotplug capable devices fatal (DPC) error recovery
>>>>>> is currently broken. Current fatal error recovery implementation
>>>>>> relies on PCIe hotplug (pciehp) handler for detaching and
>>>>>> re-enumerating the affected devices/drivers. So when dealing with
>>>>>> non-hotplug capable devices, recovery code does not restore the state
>>>>>> of the affected devices correctly. You can find more details about
>>>>>> this issue in the following links.
>>>>>>
>>>>>> https://lore.kernel.org/linux-pci/20200527083130.4137-1-Zhiqiang.Hou@nxp.com/
>>>>>> https://lore.kernel.org/linux-pci/12115.1588207324@famine/
>>>>>> https://lore.kernel.org/linux-pci/0e6f89cd6b9e4a72293cc90fafe93487d7c2d295.1585000084.git.sathyanarayanan.kuppuswamy@linux.intel.com/
>>>>>>
>>>>>> In order to fix the above two issues, we should stop relying on hotplug
>>>>>      Yes, it doesn't rely on hotplug handler to remove and rescan the device,
>>>>> but it couldn't prevent hotplug drivers from doing another replicated
>>>>> removal/rescanning.
>>>>> it doesn't make sense to leave another useless removal/rescanning there.
>>>>> Maybe that's why these two paths were merged to one and made it rely on
>>>>> hotplug.
>>>> No, as per PCIe spec, hotplug and DPC has no functional dependency. Hence
>>>> depending on it to handle some of its recovery function is in-correct and
>>>> would lead to issues in non-hotplug capable platforms (which is true
>>>> currently).
>>>>>
>>

> 
>>>    Though pciehp is not so hot/scalable and performance critical, but there
>>>    is per cpu thread to handle hot-plug operation. synchronize all threads
>>>    make them walk backwards for scalability.
>> DPC events does not happen in high frequency. So I don't think we should
>   It's holding global lock, once malfunction happens to one device and
> it's driver,
> the whole system, everyone holds it, would be blocked to work.
>> worry about the performance here. Even hotplug handler will hold this lock
>> when adding/removing the devices. So adding/removing devices is a serialized
> You don't worry about performance, but if there is a requirement needs
> more scalable
> and reliable hotplug, the effect will be much harder. what to do then ? choose
> another OS ?
As I have mentioned, all device creation/removal in PCI core code is already
protected by this lock (including hotplug code).  So the multidomain performance
impact you mentioned should exist even now. All I am doing is, using the
same lock for protecting device removal/rescan in error recovery code.

drivers/pci/xen-pcifront.c:477:	pci_lock_rescan_remove();
drivers/pci/xen-pcifront.c:567:	pci_lock_rescan_remove();
drivers/pci/xen-pcifront.c:1064:		pci_lock_rescan_remove();
drivers/pci/hotplug/rpaphp_core.c:498:		pci_lock_rescan_remove();
drivers/pci/hotplug/rpaphp_core.c:520:	pci_lock_rescan_remove();
drivers/pci/hotplug/s390_pci_hpc.c:70:	pci_lock_rescan_remove();
drivers/pci/hotplug/shpchp_pci.c:31:	pci_lock_rescan_remove();
drivers/pci/hotplug/shpchp_pci.c:73:	pci_lock_rescan_remove();
drivers/pci/hotplug/pciehp_pci.c:39:	pci_lock_rescan_remove();
drivers/pci/hotplug/pciehp_pci.c:96:	pci_lock_rescan_remove();
drivers/pci/hotplug/acpiphp_glue.c:762:		pci_lock_rescan_remove();
drivers/pci/hotplug/acpiphp_glue.c:787:	pci_lock_rescan_remove();
drivers/pci/hotplug/acpiphp_glue.c:975:	pci_lock_rescan_remove();
drivers/pci/hotplug/acpiphp_glue.c:1026:	pci_lock_rescan_remove();
drivers/pci/hotplug/cpqphp_pci.c:75:	pci_lock_rescan_remove();
drivers/pci/hotplug/cpqphp_pci.c:120:	pci_lock_rescan_remove();
drivers/pci/hotplug/rpadlpar_core.c:361:	pci_lock_rescan_remove();
drivers/pci/hotplug/pnv_php.c:513:			pci_lock_rescan_remove();
drivers/pci/hotplug/pnv_php.c:582:	pci_lock_rescan_remove();
drivers/pci/hotplug/ibmphp_core.c:668:	pci_lock_rescan_remove();
drivers/pci/hotplug/ibmphp_core.c:738:	pci_lock_rescan_remove();
drivers/pci/hotplug/cpci_hotplug_pci.c:245:	pci_lock_rescan_remove();
drivers/pci/hotplug/cpci_hotplug_pci.c:298:	pci_lock_rescan_remove();
drivers/pci/controller/pci-hyperv.c:1866:	pci_lock_rescan_remove();
drivers/pci/controller/pci-hyperv.c:2135:		pci_lock_rescan_remove();
drivers/pci/controller/pci-hyperv.c:2313:		pci_lock_rescan_remove();
drivers/pci/controller/pci-hyperv.c:3300:		pci_lock_rescan_remove();
drivers/pci/controller/pci-host-common.c:91:	pci_lock_rescan_remove();
drivers/pci/remove.c:123:	pci_lock_rescan_remove();
drivers/pci/pci-sysfs.c:410:		pci_lock_rescan_remove();
drivers/pci/pci-sysfs.c:444:		pci_lock_rescan_remove();
drivers/pci/pci-sysfs.c:479:		pci_lock_rescan_remove();
drivers/pci/probe.c:3231:void pci_lock_rescan_remove(void)
drivers/pci/probe.c:3235:EXPORT_SYMBOL_GPL(pci_lock_rescan_remove);

> To be honest, I don't like the global lock/ pci_lock_rescan_remove().
> 
> BTW, I didn't try the FATAL errors brute force injection on your
> patch, duplicated
> removal will work naturally because it was removed ?
> 
> Thanks,
> Ethan
>> operation.
>>>
>>
>>>>
>>>>>> --
>>>>>> 2.17.1
>>>>>>
>>>>
>>>> --
>>>> Sathyanarayanan Kuppuswamy
>>>> Linux Kernel Developer
>>
>> --
>> Sathyanarayanan Kuppuswamy
>> Linux Kernel Developer

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer