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 commit61ec7567db
(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 commit8b3b295502
(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 <hugh@veritas.com> Cc: Alexey Dobriyan <adobriyan@sw.ru> Cc: Len Brown <lenb@kernel.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Jan Beulich <jbeulich@novell.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -397,10 +397,6 @@ static void __init smp_init(void)
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{
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unsigned int cpu;
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#ifndef CONFIG_HOTPLUG_CPU
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cpu_possible_map = cpu_present_map;
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#endif
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/* FIXME: This should be done in userspace --RR */
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for_each_present_cpu(cpu) {
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if (num_online_cpus() >= max_cpus)
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@ -545,10 +541,6 @@ asmlinkage void __init start_kernel(void)
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setup_arch(&command_line);
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setup_command_line(command_line);
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unwind_setup();
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#ifndef CONFIG_HOTPLUG_CPU
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if (max_cpus < 2)
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cpu_possible_map = cpu_online_map;
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#endif
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setup_per_cpu_areas();
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smp_prepare_boot_cpu(); /* arch-specific boot-cpu hooks */
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