RE: HW power fault defect cause system hang on kernel 5.4.y

From: Bao, Joseph
Date: Wed Nov 10 2021 - 04:23:07 EST

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Hi Stuart,
The patch you attached does not work for me, the logic should be ok but I have not figured out why. But a loop counter actually helps mitigate this issue. Thank you very much!

diff --git a/drivers/pci/hotplug/pciehp_hpc.c b/drivers/pci/hotplug/pciehp_hpc.c
index 88b996764ff9..3d2c336ff740 100644
--- a/drivers/pci/hotplug/pciehp_hpc.c
+++ b/drivers/pci/hotplug/pciehp_hpc.c
@@ -529,7 +529,8 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
struct controller *ctrl = (struct controller *)dev_id;
struct pci_dev *pdev = ctrl_dev(ctrl);
struct device *parent = pdev->dev.parent;
- u16 status, events = 0;
+ u16 status, events, read_retry_count = 0;
+ u8 READ_RETRY_MAX = 6;

/*
* Interrupts only occur in D3hot or shallower and only if enabled
@@ -585,7 +586,7 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
return IRQ_NONE;
}

- if (status) {
+ if (status && (read_retry_count < READ_RETRY_MAX)) {
pcie_capability_write_word(pdev, PCI_EXP_SLTSTA, events);

/*
@@ -594,8 +595,10 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
* So re-read the Slot Status register in case a bit was set
* between read and write.
*/
- if (pci_dev_msi_enabled(pdev) && !pciehp_poll_mode)
+ if (pci_dev_msi_enabled(pdev) && !pciehp_poll_mode) {
+ read_retry_count++;
goto read_status;
+ }
}
Regards.
Joseph

Regards.
Joseph

-----Original Message-----
From: stuart hayes <stuart.w.hayes@xxxxxxxxx>
Sent: Tuesday, November 9, 2021 11:37 PM
To: Bao, Joseph <joseph.bao@xxxxxxxxx>; Bjorn Helgaas <helgaas@xxxxxxxxxx>
Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>; linux-pci@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Lukas Wunner <lukas@xxxxxxxxx>
Subject: Re: HW power fault defect cause system hang on kernel 5.4.y

On 11/9/2021 1:59 AM, Bao, Joseph wrote:
> Hi Lukas/Stuart,
> Want to follow up with you whether the system hang is expected when HW has a defect keeping PCI_EXP_SLTSTA_PFD always HIGH.
>
>
> Regards
> Joseph
>

It does appear that the code will hang when pciehp_isr sees PFD high and power_fault_detected isn't yet set, if PFD doesn't clear when a 1 is written to it. It will continue to loop trying to clear it, and power_fault_detected won't get set until after it gets through this loop.

It wouldn't be hard to modify that code to only attempt to clear each bit once. I wouldn't expect the same event bit to get set twice within this loop, so this might fix it (I did not test). Alternately, a loop counter could be added to prevent it from looping more than some arbitrary number
(6?) of times in case of stuck bits.

diff --git a/drivers/pci/hotplug/pciehp_hpc.c b/drivers/pci/hotplug/pciehp_hpc.c
index 3024d7e85e6a..3e502b4e8ef7 100644
--- a/drivers/pci/hotplug/pciehp_hpc.c
+++ b/drivers/pci/hotplug/pciehp_hpc.c
@@ -594,7 +594,7 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
struct controller *ctrl = (struct controller *)dev_id;
struct pci_dev *pdev = ctrl_dev(ctrl);
struct device *parent = pdev->dev.parent;
- u16 status, events = 0;
+ u16 changed, status, events = 0;

/*
* Interrupts only occur in D3hot or shallower and only if enabled @@ -643,6 +643,7 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
if (ctrl->power_fault_detected)
status &= ~PCI_EXP_SLTSTA_PFD;

+ changed = status ^ events;
events |= status;
if (!events) {
if (parent)
@@ -659,7 +660,7 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
* So re-read the Slot Status register in case a bit was set
* between read and write.
*/
- if (pci_dev_msi_enabled(pdev) && !pciehp_poll_mode)
+ if (pci_dev_msi_enabled(pdev) && !pciehp_poll_mode && changed)
goto read_status;
}

> -----Original Message-----
> From: Bjorn Helgaas <helgaas@xxxxxxxxxx>> Sent: Wednesday, November 3,
> 2021 6:34 AM
> To: Bao, Joseph <joseph.bao@xxxxxxxxx>
> Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>; linux-pci@xxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; Stuart Hayes <stuart.w.hayes@xxxxxxxxx>;
> Lukas Wunner <lukas@xxxxxxxxx>
> Subject: Re: HW power fault defect cause system hang on kernel 5.4.y
>
> [+cc Stuart, author of 8edf5332c393 ("PCI: pciehp: Fix MSI interrupt
> race"), Lukas, pciehp expert]
>
> On Tue, Nov 02, 2021 at 03:45:00AM +0000, Bao, Joseph wrote:
>> Hi, dear kernel developer,
>>
>> Recently we encounter system hang (dead spinlock) when move to kernel
>> linux-5.4.y.
>>
>> Finally, we use bisect to locate the suspicious commit
>> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-5.4.y&id=4667358dab9cc07da044d5bc087065545b1000df.
>
> 4667358dab9c backported upstream commit 8edf5332c393 ("PCI: pciehp:
> Fix MSI interrupt race") to v5.4.69 just over a year ago.
>
>> Our system has some HW defect, which will wrongly set
>> PCI_EXP_SLTSTA_PFD high, and this commit will lead to infinite loop
>> jumping to read_status (no chance to clear status PCI_EXP_SLTSTA_PFD
>> bit since ctrl is not updated), I know this is our HW defect, but
>> this commit makes kernel trapped in this isr function and leads to
>> kernel hang (then the user could not get useful information to show
>> what's wrong), which I think is not expected behavior, so I would
>> like to report to you for discussion.
>
> I guess this happens because the first time we handle PFD,
> pciehp_ist() sets "ctrl->power_fault_detected = 1", and when power_fault_detected is set, pciehp_isr() won't clear PFD from PCI_EXP_SLTSTA?
>
> It looks like the only place we clear power_fault_detected is in pciehp_power_on_slot(), and I don't think we call that unless we have a presence detect or link status change.
>
> It would definitely be nice if we could arrange so this hardware defect didn't cause a kernel hang.
>
> I think the diff below is the backport of 8edf5332c393 ("PCI: pciehp:
> Fix MSI interrupt race").
>
>> diff --git a/drivers/pci/hotplug/pciehp_hpc.c
>> b/drivers/pci/hotplug/pciehp_hpc.c
>> index 356786a3b7f4b..88b996764ff95 100644
>> ---
>> a/https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tr
>> e
>> e/drivers/pci/hotplug/pciehp_hpc.c?h=linux-5.4.y&id=ca767cf0152d18fc2
>> 9
>> 9cde85b18d1f46ac21e1ba
>> +++ b/https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.gi
>> +++ t
>> +++ /tree/drivers/pci/hotplug/pciehp_hpc.c?h=linux-5.4.y&id=4667358da
>> +++ b
>> +++ 9cc07da044d5bc087065545b1000df
>> @@ -529,7 +529,7 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
>> struct controller *ctrl = (struct controller *)dev_id;
>> struct pci_dev *pdev = ctrl_dev(ctrl);
>> struct device *parent = pdev->dev.parent;
>> - u16 status, events;
>> + u16 status, events = 0;
>>
>> /*
>> * Interrupts only occur in D3hot or shallower and only if enabled
>> @@ -554,6 +554,7 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
>> }
>> }
>>
>> +read_status:
>> pcie_capability_read_word(pdev, PCI_EXP_SLTSTA, &status);
>> if (status == (u16) ~0) {
>> ctrl_info(ctrl, "%s: no response from device\n", __func__); @@
>> -566,24 +567,37 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
>> * Slot Status contains plain status bits as well as event
>> * notification bits; right now we only want the event bits.
>> */
>> - events = status & (PCI_EXP_SLTSTA_ABP | PCI_EXP_SLTSTA_PFD |
>> - PCI_EXP_SLTSTA_PDC | PCI_EXP_SLTSTA_CC |
>> - PCI_EXP_SLTSTA_DLLSC);
>> + status &= PCI_EXP_SLTSTA_ABP | PCI_EXP_SLTSTA_PFD |
>> + PCI_EXP_SLTSTA_PDC | PCI_EXP_SLTSTA_CC |
>> + PCI_EXP_SLTSTA_DLLSC;
>>
>> /*
>> * If we've already reported a power fault, don't report it again
>> * until we've done something to handle it.
>> */
>> if (ctrl->power_fault_detected)
>> - events &= ~PCI_EXP_SLTSTA_PFD;
>> + status &= ~PCI_EXP_SLTSTA_PFD;
>>
>> + events |= status;
>> if (!events) {
>> if (parent)
>> pm_runtime_put(parent);
>> return IRQ_NONE;
>> }
>>
>> - pcie_capability_write_word(pdev, PCI_EXP_SLTSTA, events);
>> + if (status) {
>> + pcie_capability_write_word(pdev, PCI_EXP_SLTSTA, events);
>> +
>> + /*
>> + * In MSI mode, all event bits must be zero before the port
>> + * will send a new interrupt (PCIe Base Spec r5.0 sec 6.7.3.4).
>> + * So re-read the Slot Status register in case a bit was set
>> + * between read and write.
>> + */
>> + if (pci_dev_msi_enabled(pdev) && !pciehp_poll_mode)
>> + goto read_status;
>> + }
>> +
>> ctrl_dbg(ctrl, "pending interrupts %#06x from Slot Status\n", events);
>> if (parent)
>> pm_runtime_put(parent);

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In reply to: stuart hayes: "Re: HW power fault defect cause system hang on kernel 5.4.y"
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