Re: [PATCH v4] PCI: pciehp: Fix MSI interrupt race

[Bjorn Helgaas]

On Wed, Feb 19, 2020 at 03:31:13PM +0100, Lukas Wunner wrote:
> From: Stuart Hayes <stuart.w.hayes@xxxxxxxxx>
> 
> Without this commit, a PCIe hotplug port can stop generating interrupts
> on hotplug events, so device adds and removals will not be seen:
> 
> The pciehp interrupt handler pciehp_isr() reads the Slot Status register
> and then writes back to it to clear the bits that caused the interrupt.
> If a different interrupt event bit gets set between the read and the
> write, pciehp_isr() returns without having cleared all of the interrupt
> event bits.  If this happens when the MSI isn't masked (which by default
> it isn't in handle_edge_irq(), and which it will never be when MSI
> per-vector masking is not supported), we won't get any more hotplug
> interrupts from that device.
> 
> That is expected behavior, according to the PCIe Base Spec r5.0, section
> 6.7.3.4, "Software Notification of Hot-Plug Events".
> 
> Because the Presence Detect Changed and Data Link Layer State Changed
> event bits can both get set at nearly the same time when a device is
> added or removed, this is more likely to happen than it might seem.
> The issue was found (and can be reproduced rather easily) by connecting
> and disconnecting an NVMe storage device on at least one system model
> where the NVMe devices were being connected to an AMD PCIe port (PCI
> device 0x1022/0x1483).
> 
> Fix the issue by modifying pciehp_isr() to loop back and re-read the
> Slot Status register immediately after writing to it, until it sees that
> all of the event status bits have been cleared.
> 
> Signed-off-by: Stuart Hayes <stuart.w.hayes@xxxxxxxxx>
> [lukas: drop loop count limitation, write "events" instead of "status",
> don't loop back in INTx and poll modes, tweak code comment & commit msg]
> Signed-off-by: Lukas Wunner <lukas@xxxxxxxxx>

Applied to pci/hotplug for v5.7, thanks!

> ---
> v4 (lukas):
>   * drop "MAX_ISR_STATUS_READS" loop count limitation
>   * drop unnecessary braces around PCI_EXP_SLTSTA_* flags
>   * write "events" instead of "status" variable to Slot Status register
>     to avoid unnecessary loop iterations if the same bit gets set
>     repeatedly
>   * don't loop back in INTx and poll modes
>   * shorten and tweak code comment & commit message
> 
> v3:
>   * removed pvm_capable flag (from v2) since MSI may not be masked
>     regardless of whether per-vector masking is supported
>   * tweaked comments
> 
> v2:
>   * fixed ctrl_warn() call
>   * improved comments
>   * added pvm_capable flag and changed pciehp_isr() to loop back only when
>     pvm_capable flag not set (suggested by Lukas Wunner)
> 
>  drivers/pci/hotplug/pciehp_hpc.c | 26 ++++++++++++++++++++------
>  1 file changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/pci/hotplug/pciehp_hpc.c b/drivers/pci/hotplug/pciehp_hpc.c
> index 8a2cb1764386..f64d10df9eb5 100644
> --- a/drivers/pci/hotplug/pciehp_hpc.c
> +++ b/drivers/pci/hotplug/pciehp_hpc.c
> @@ -527,7 +527,7 @@ 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
> @@ -552,6 +552,7 @@ 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__);
> @@ -564,24 +565,37 @@ 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);
> -- 
> 2.24.0
>