Re: [PATCH v3 1/6] PCI: hv: Fix a race condition bug in hv_pci_query_relations()
From: Lorenzo Pieralisi <lpieralisi@kernel.org>
Date: 2023-05-25 08:14:54
Also in:
linux-hyperv, linux-pci, linux-rdma, lkml, stable
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.
quoted hunk ↗ jump to hunk
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 <redacted> 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survey_event), + * even after hv_pci_query_relations() exits and the stack variable + * 'comp' is no longer valid.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 I would remove it - given that you already explain that comp is no longer valid - that is already a bug that needs fixing.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