From 62e6f1e8bb7c48c02b8bdb3085c5f6365682149b Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Fri, 31 Aug 2007 04:26:50 +0100 Subject: [PATCH] fix maxcpus=1 oops in show_stat() Alexey Dobriyan reports that maxcpus=1 is still broken in 2.6.23-rc4: if CONFIG_HOTPLUG_CPU is not set, x86_64 bootup oopses in show_stat() - for_each_possible_cpu accesses a per-cpu area which was never set up. Alexey identified commit 61ec7567db103d537329b0db9a887db570431ff4 (ACPI: boot correctly with "nosmp" or "maxcpus=0") as the origin; but it's not really to blame, just exposes a bug in 2.6.23-rc1's commit 8b3b295502444340dd0701855ac422fbf32e161d (Especially when !CONFIG_HOTPLUG_CPU, avoid needlessy allocating resources for CPUs that can never become available). rc1's test for max_cpus < 2 in start_kernel() wasn't working because max_cpus was still NR_CPUS at that point: until rc4 moved the maxcpus parsing earlier. Now it sets cpu_possible_map to 1 before allocating all possible per-cpu areas; then smp_init() expands cpu_possible_map to cpu_present_map (0xf in my case) later on. rc1's commit has good intentions, but expects cpu_present_map to be limited by maxcpus, which is only the case on i386. cpus_and(possible, possible,present) might be good, but needs an audit of cpu_present_map uses - there may well be assumptions that any cpu present is possible. So stay safe for now and just revert those #ifndef CONFIG_HOTPLUG_CPU optimizations in rc1's commit. Signed-off-by: Hugh Dickins Cc: Alexey Dobriyan Cc: Len Brown Cc: Andrew Morton Cc: Jan Beulich Signed-off-by: Linus Torvalds --- init/main.c | 8 -------- 1 file changed, 8 deletions(-) diff --git a/init/main.c b/init/main.c index 450e6ee52a..9def935ab1 100644 --- a/init/main.c +++ b/init/main.c @@ -397,10 +397,6 @@ static void __init smp_init(void) { unsigned int cpu; -#ifndef CONFIG_HOTPLUG_CPU - cpu_possible_map = cpu_present_map; -#endif - /* FIXME: This should be done in userspace --RR */ for_each_present_cpu(cpu) { if (num_online_cpus() >= max_cpus) @@ -545,10 +541,6 @@ asmlinkage void __init start_kernel(void) setup_arch(&command_line); setup_command_line(command_line); unwind_setup(); -#ifndef CONFIG_HOTPLUG_CPU - if (max_cpus < 2) - cpu_possible_map = cpu_online_map; -#endif setup_per_cpu_areas(); smp_prepare_boot_cpu(); /* arch-specific boot-cpu hooks */ -- 2.39.5