From: Lorenzo Pieralisi lpieralisi@kernel.org Sent: Thursday, May 25, 2023 1:14 AM To: Dexuan Cui decui@microsoft.com
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.
Hi Lozenzo, I'm sorry for the late response -- finally I'm back on the patchset...
I'll use the below commit message in v4:
Since day 1 of the driver, there has been a race between hv_pci_query_relations() and survey_child_resources(): during fast device hotplug, hv_pci_query_relations() may error out due to device-remove and the stack variable 'comp' is no longer valid; however, pci_devices_present_work() -> survey_child_resources() -> complete() may be running on another CPU and accessing the no-longer-valid 'comp'. Fix the race by flushing the workqueue before we exit from hv_pci_query_relations().
I'll also update the comment to share more details of the race:
--- a/drivers/pci/controller/pci-hyperv.c +++ b/drivers/pci/controller/pci-hyperv.c @@ -3401,6 +3401,24 @@ 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 pci_devices_present_work() -> survey_child_resources() -> + * complete(&hbus->survey_event), even after hv_pci_query_relations() + * exits and the stack variable 'comp' is no longer valid; as a result, + * a hang or a page fault may happen when the complete() calls + * raw_spin_lock_irqsave(). Flush hbus->wq before we exit from + * hv_pci_query_relations() to avoid the issues. Note: if 'ret' is + * -ENODEV, there can't be any more work item scheduled to hbus->wq + * after the flush_workqueue(): see vmbus_onoffer_rescind() -> + * vmbus_reset_channel_cb(), vmbus_rescind_cleanup() -> + * channel->rescind = true. + */ + flush_workqueue(hbus->wq); + return ret; }
* 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 canbe
* 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.
I share more details in the comment (see the above).
Acked-by: Lorenzo Pieralisi lpieralisi@kernel.org
Thanks!