[PATCH v2 1/6] PCI: hv: Fix a race condition bug in hv_pci_query_relations()

From: Dexuan Cui <decui@microsoft.com>
To: bhelgaas@google.com, davem@davemloft.net, decui@microsoft.com,
	edumazet@google.com, haiyangz@microsoft.com, jakeo@microsoft.com,
	kuba@kernel.org, kw@linux.com, kys@microsoft.com,
	leon@kernel.org, linux-pci@vger.kernel.org,
	lpieralisi@kernel.org, mikelley@microsoft.com, pabeni@redhat.com,
	robh@kernel.org, saeedm@nvidia.com, wei.liu@kernel.org,
	longli@microsoft.com, boqun.feng@gmail.com,
	ssengar@microsoft.com, helgaas@kernel.org
Cc: linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-rdma@vger.kernel.org, netdev@vger.kernel.org,
	stable@vger.kernel.org
Subject: [PATCH v2 1/6] PCI: hv: Fix a race condition bug in hv_pci_query_relations()
Date: Mon,  3 Apr 2023 19:05:40 -0700	[thread overview]
Message-ID: <20230404020545.32359-2-decui@microsoft.com> (raw)
In-Reply-To: <20230404020545.32359-1-decui@microsoft.com>

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().

Fixes: 4daace0d8ce8 ("PCI: hv: Add paravirtual PCI front-end for Microsoft Hyper-V VMs")
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Cc: stable@vger.kernel.org
---

v2: 
  Removed the "debug code".
  No change to the patch body.
  Added Cc:stable

 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+	 * 'comp' is no longer valid.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
+	 * 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

