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To:     Dexuan Cui <decui@microsoft.com>,
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Subject: RE: [PATCH v2 1/6] PCI: hv: Fix a race condition bug in
 hv_pci_query_relations()
Thread-Topic: [PATCH v2 1/6] PCI: hv: Fix a race condition bug in
 hv_pci_query_relations()
Thread-Index: AQHZZpomvbk4R6YhiUi3EzsVimhRK68gB8Zw
Date:   Fri, 7 Apr 2023 16:03:57 +0000
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References: <20230404020545.32359-1-decui@microsoft.com>
 <20230404020545.32359-2-decui@microsoft.com>
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From: Dexuan Cui <decui@microsoft.com> Sent: Monday, April 3, 2023 7:06 PM
>=20
> 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().
>=20
> Fixes: 4daace0d8ce8 ("PCI: hv: Add paravirtual PCI front-end for Microsof=
t Hyper-V
> VMs")
> Signed-off-by: Dexuan Cui <decui@microsoft.com>
> Cc: stable@vger.kernel.org
> ---
>=20
> v2:
>   Removed the "debug code".
>   No change to the patch body.
>   Added Cc:stable
>=20
>  drivers/pci/controller/pci-hyperv.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>=20
> 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 =3D wait_for_response(hdev, &comp);
>=20
> +	/*
> +	 * 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 +	 * 'comp' is no longer valid.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
> +	 * 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;
>  }
>=20
> --
> 2.25.1

Reviewed-by: Michael Kelley <mikelley@microsoft.com>