Re: [PATCH v3] PCI: hv: Warn when wait_for_response() waits indefinitely

[Manivannan Sadhasivam <mani@kernel.org>]

On Wed, Sep 09, 2026 at 03:01:22PM +0530, Naman Jain wrote:
> 
> 
> On 9/8/2026 5:00 PM, 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 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: Michael Kelley <mhklinux@outlook.com>
> > Signed-off-by: Sahil Chandna <sahilchandna@linux.microsoft.com>
> > ---
> > Changes since v2:
> > - Add counter based timeout instead of introducing another timer
> > Link to v2: https://lore.kernel.org/all/20260902115854.2629164-1-sahilchandna@linux.microsoft.com/
> > 
> > Changes since v1:
> > - Removed periodic warning to one time warning in 2 minutes
> > - Include vmbus relid and stuck PCI msg.
> > Link to v1: https://lore.kernel.org/all/20260825051850.2438816-1-sahilchandna@linux.microsoft.com/
> >   drivers/pci/controller/pci-hyperv.c | 46 ++++++++++++++++++++++-------
> >   1 file changed, 36 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> > index 89816a2bd7cd..bd07402c47ed 100644
> > --- a/drivers/pci/controller/pci-hyperv.c
> > +++ b/drivers/pci/controller/pci-hyperv.c
> > @@ -1040,19 +1040,38 @@ 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 2 minutes
> > + * without either a rescind or completion, add a warning.
> >    */
> > +#define PCI_RESPONSE_HANG_TICKS 1200
> > +
> 
> Nit, I think this variable name could be misleading, as this represents a
> loop counter for a retry logic, but not exactly *ticks* from kernel
> terminology.
> 
> I would have preferred something like: PCI_RESPONSE_WARN_POLL_COUNT.
> 

PCI_RESPONSE_POLL_COUNT?

- Mani

-- 
மணிவண்ணன் சதாசிவம்