Re: [PATCH] PCI: hv: Warn when wait_for_response() waits indefinitely

[Naman Jain <namjain@linux.microsoft.com>]

On 8/25/2026 10:47 AM, Sahil Chandna wrote:
> A guest can wait indefinitely in wait_for_response() for the host to
> send either a rescind message or a packet completion. If the
> host does not send either, the guest can remain blocked with no
> diagnostic indicating a reason.
> This was observed during a guest kernel upgrade in which the
> host-side application handling the PCI channel faulted, causing the
> guest to never receive the completion request.
> Add a periodic warning in wait_for_response() when the wait exceeds
> a timeout so that such a hang is visible in the guest's kernel log
> and can be correlated with host-side state.
> 
> Suggested-by: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
> Signed-off-by: Sahil Chandna <sahilchandna@linux.microsoft.com>
> ---
> This was sent earlier upstream [1]
> [1] https://lore.kernel.org/linux-hyperv/20260612174010.2598695-1-hamzamahfooz@linux.microsoft.com/
> ---
>   drivers/pci/controller/pci-hyperv.c | 14 +++++++++++++-
>   1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> index cfc8fa403dad..c4fba0039164 100644
> --- a/drivers/pci/controller/pci-hyperv.c
> +++ b/drivers/pci/controller/pci-hyperv.c
> @@ -1040,11 +1040,18 @@ static void put_pcichild(struct hv_pci_dev *hpdev)
> 
>   /*
>    * There is no good way to get notified from vmbus_onoffer_rescind(),
> - * so let's use polling here, since this is not a hot path.
> + * so let's use polling here, since this is not a hot path. If
> + * wait_for_response() has been polling for PCI_RESPONSE_HANG_TIMEOUT_SEC
> + * without either a rescind or completion, add a periodic warning.
>    */
> +#define PCI_RESPONSE_HANG_TIMEOUT_SEC 300
> +
>   static int wait_for_response(struct hv_device *hdev,
>   			     struct completion *comp)
>   {
> +	unsigned long delay = secs_to_jiffies(PCI_RESPONSE_HANG_TIMEOUT_SEC);
> +	u64 timeout = get_jiffies_64() + delay;
> +
>   	while (true) {
>   		if (hdev->channel->rescind) {
>   			dev_warn_once(&hdev->device, "The device is gone.\n");
> @@ -1053,6 +1060,11 @@ static int wait_for_response(struct hv_device *hdev,
> 
>   		if (wait_for_completion_timeout(comp, HZ / 10))
>   			break;
> +
> +		if (time_after64(get_jiffies_64(), timeout)) {
> +			dev_warn(&hdev->device, "PCI stuck waiting for response.\n");
> +			timeout = get_jiffies_64() + delay;
> +		}
>   	}

There can be some enhancements in above patch to address these problems:
1. Logging forever every 5 minutes in case of no completion or rescind.
2. If we now print warning once, not knowing if completion ever arrived.
3. On solving pt. 1 and 2 by adding a print for completion, one should 
avoid adding a print by default for regular timely completions.

Basically something like this:

#define PCI_RESPONSE_WARN_TIMEOUT_SEC 300

static int wait_for_response(struct hv_device *hdev,
                  struct completion *comp)
{
     unsigned long warn_at =
         jiffies + secs_to_jiffies(PCI_RESPONSE_WARN_TIMEOUT_SEC);
     bool warned = false;

     while (true) {
         if (hdev->channel->rescind) {
             dev_warn_once(&hdev->device, "The device is gone.\n");
             return -ENODEV;
         }

         if (wait_for_completion_timeout(comp, HZ / 10)) {
             if (warned || time_after_eq(jiffies, warn_at))
                 dev_warn(&hdev->device,
                      "PCI response received after prolonged wait.\n");
             return 0;
         }

         if (!warned && time_after_eq(jiffies, warn_at)) {
             dev_warn(&hdev->device,
                  "PCI still waiting for response.\n");
             warned = true;
         }
     }
}

Regards,
Naman