• Hugh Dickins's avatar
    fix maxcpus=1 oops in show_stat() · 62e6f1e8
    Hugh Dickins authored
    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 61ec7567
    (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
     (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: default avatarHugh 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: default avatarLinus Torvalds <torvalds@linux-foundation.org>
main.c 20.6 KB