From 03bd4e1f7265548832a76e7919a81f3137c44fd1 Mon Sep 17 00:00:00 2001 From: Wanpeng Li Date: Wed, 24 Sep 2014 16:38:05 +0800 Subject: [PATCH] sched: Fix unreleased llc_shared_mask bit during CPU hotplug The following bug can be triggered by hot adding and removing a large number of xen domain0's vcpus repeatedly: BUG: unable to handle kernel NULL pointer dereference at 0000000000000004 IP: [..] find_busiest_group PGD 5a9d5067 PUD 13067 PMD 0 Oops: 0000 [#3] SMP [...] Call Trace: load_balance ? _raw_spin_unlock_irqrestore idle_balance __schedule schedule schedule_timeout ? lock_timer_base schedule_timeout_uninterruptible msleep lock_device_hotplug_sysfs online_store dev_attr_store sysfs_write_file vfs_write SyS_write system_call_fastpath Last level cache shared mask is built during CPU up and the build_sched_domain() routine takes advantage of it to setup the sched domain CPU topology. However, llc_shared_mask is not released during CPU disable, which leads to an invalid sched domainCPU topology. This patch fix it by releasing the llc_shared_mask correctly during CPU disable. Yasuaki also reported that this can happen on real hardware: https://lkml.org/lkml/2014/7/22/1018 His case is here: == Here is an example on my system. My system has 4 sockets and each socket has 15 cores and HT is enabled. In this case, each core of sockes is numbered as follows: | CPU# Socket#0 | 0-14 , 60-74 Socket#1 | 15-29, 75-89 Socket#2 | 30-44, 90-104 Socket#3 | 45-59, 105-119 Then llc_shared_mask of CPU#30 has 0x3fff80000001fffc0000000. It means that last level cache of Socket#2 is shared with CPU#30-44 and 90-104. When hot-removing socket#2 and #3, each core of sockets is numbered as follows: | CPU# Socket#0 | 0-14 , 60-74 Socket#1 | 15-29, 75-89 But llc_shared_mask is not cleared. So llc_shared_mask of CPU#30 remains having 0x3fff80000001fffc0000000. After that, when hot-adding socket#2 and #3, each core of sockets is numbered as follows: | CPU# Socket#0 | 0-14 , 60-74 Socket#1 | 15-29, 75-89 Socket#2 | 30-59 Socket#3 | 90-119 Then llc_shared_mask of CPU#30 becomes 0x3fff8000fffffffc0000000. It means that last level cache of Socket#2 is shared with CPU#30-59 and 90-104. So the mask has the wrong value. Signed-off-by: Wanpeng Li Tested-by: Linn Crosetto Reviewed-by: Borislav Petkov Reviewed-by: Toshi Kani Reviewed-by: Yasuaki Ishimatsu Cc: Cc: David Rientjes Cc: Prarit Bhargava Cc: Steven Rostedt Cc: Peter Zijlstra Link: http://lkml.kernel.org/r/1411547885-48165-1-git-send-email-wanpeng.li@linux.intel.com Signed-off-by: Ingo Molnar --- arch/x86/kernel/smpboot.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c index 2d872e08fab9..42a2dca984b3 100644 --- a/arch/x86/kernel/smpboot.c +++ b/arch/x86/kernel/smpboot.c @@ -1284,6 +1284,9 @@ static void remove_siblinginfo(int cpu) for_each_cpu(sibling, cpu_sibling_mask(cpu)) cpumask_clear_cpu(cpu, cpu_sibling_mask(sibling)); + for_each_cpu(sibling, cpu_llc_shared_mask(cpu)) + cpumask_clear_cpu(cpu, cpu_llc_shared_mask(sibling)); + cpumask_clear(cpu_llc_shared_mask(cpu)); cpumask_clear(cpu_sibling_mask(cpu)); cpumask_clear(cpu_core_mask(cpu)); c->phys_proc_id = 0; -- 2.20.1