SRU request sent to the kernel team mailing list: https://lists.ubuntu.com/archives/kernel-team/2019-February/098872.html
-- You received this bug notification because you are a member of Kernel Packages, which is subscribed to linux in Ubuntu. https://bugs.launchpad.net/bugs/1817918 Title: Hard lockups due to unrestricted lapic timer delay Status in linux package in Ubuntu: Confirmed Status in linux source package in Xenial: Confirmed Status in linux source package in Bionic: Fix Released Bug description: [Impact] * There is a long-time report of an issue with the TSC delay present in wait_lapic_expire() - basically the guest could have an expiration timer configured in a way it induces host to wait a long time (with preemption disabled), so there's a potential scenario for host lockups. * The stack trace we have access (from an user report of this issue) is (summarized) below: NMI watchdog: Watchdog detected hard LOCKUP on cpu 16 [...] CPU: 16 PID: 3024910 Comm: CPU 0/KVM Not tainted 4.4.0-139-generic #165-Ubuntu RIP: 0010:[<addr>] [<addr>] delay_tsc+0x20/0x60 [...] __delay+0x15/0x20 wait_lapic_expire+0xc3/0x150 [kvm] vcpu_enter_guest+0x743/0x11d0 [kvm] kvm_arch_vcpu_ioctl_run+0xe6/0x410 [kvm] kvm_vcpu_ioctl+0x33d/0x620 [kvm] do_vfs_ioctl+0x2af/0x4b0 ? __do_page_fault+0x1c1/0x410 ? fire_user_return_notifiers+0x3e/0x50 SyS_ioctl+0x79/0x90 entry_SYSCALL_64_fastpath+0x22/0xc1 This matches the reported problem in the KVM mailing-list: https://marc.info/?l=kvm&m=146374488028339 * A fix was proposed in the above thread, but discarded in favor of the following approach: https://marc.info/?l=kvm&m=146647260109315 The patch was merged in Linus tree, hence we hereby request the SRU: b606f189c7d5 ("KVM: LAPIC: cap __delay at lapic_timer_advance_ns"). There's one additional patch needed, which is just the header adjustment for exporting a necessary function. * The patch is missing only in 4.4 kernel series; Bionic (4.15) and the other newer releases have the patch already. [Test Case] * Unfortunately this is a hard to reproduce issue; we have reports of this lockup from an user, hence the SRU request here. Also, the patch was introduced originally in kernel 4.7, approx. 2.5 years ago. So, we are confident that community is running this code long enough without errors reported. Also, checked in the Linus tree and no fixes for this code were introduced since kernel 4.7. [Regression Potential] * The code modification requested here affects the amount of delay in a specific timer; the patch introduces a maximum time for delay, preventing unbounded delays in host. The regression potential is considered low, and given the nature of the modification, latency issues in guests are likely to be the most problematic regression potential we have. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1817918/+subscriptions -- Mailing list: https://launchpad.net/~kernel-packages Post to : kernel-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~kernel-packages More help : https://help.launchpad.net/ListHelp
