Re: [PATCH v3 1/6] PCI: hv: Fix a race condition bug in hv_pci_query_relations()

[Lorenzo Pieralisi <lpieralisi@kernel.org>]

On Wed, Apr 19, 2023 at 07:40:32PM -0700, Dexuan Cui wrote:
> Fix the longstanding race between hv_pci_query_relations() and
> survey_child_resources() by flushing the workqueue before we exit from
> hv_pci_query_relations().

"Fix the longstanding race" is vague. Please describe the race
succinctly at least to give an idea of what the problem is.

> Fixes: 4daace0d8ce8 ("PCI: hv: Add paravirtual PCI front-end for Microsoft Hyper-V VMs")
> Signed-off-by: Dexuan Cui <decui@microsoft.com>
> Reviewed-by: Michael Kelley <mikelley@microsoft.com>
> Cc: stable@vger.kernel.org
> ---
> 
> v2:
>   Removed the "debug code".
>   No change to the patch body.
>   Added Cc:stable
> 
> v3:
>   Added Michael's Reviewed-by.
> 
>  drivers/pci/controller/pci-hyperv.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> index f33370b756283..b82c7cde19e66 100644
> --- a/drivers/pci/controller/pci-hyperv.c
> +++ b/drivers/pci/controller/pci-hyperv.c
> @@ -3308,6 +3308,19 @@ static int hv_pci_query_relations(struct hv_device *hdev)
>  	if (!ret)
>  		ret = wait_for_response(hdev, &comp);
>  
> +	/*
> +	 * In the case of fast device addition/removal, it's possible that
> +	 * vmbus_sendpacket() or wait_for_response() returns -ENODEV but we
> +	 * already got a PCI_BUS_RELATIONS* message from the host and the
> +	 * channel callback already scheduled a work to hbus->wq, which can be
> +	 * running survey_child_resources() -> complete(&hbus->survey_event),
> +	 * even after hv_pci_query_relations() exits and the stack variable
> +	 * 'comp' is no longer valid. This can cause a strange hang issue

"A strange hang" sounds like we don't understand what's happening, it
does not feel like it is a solid understanding of the issue.

I would remove it - given that you already explain that comp is no
longer valid - that is already a bug that needs fixing.

Acked-by: Lorenzo Pieralisi <lpieralisi@kernel.org>

> +	 * or sometimes a page fault. Flush hbus->wq before we exit from
> +	 * hv_pci_query_relations() to avoid the issues.
> +	 */
> +	flush_workqueue(hbus->wq);
> +
>  	return ret;
>  }
>  
> -- 
> 2.25.1
>