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Hugh Dickins 62e6f1e8bb 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 61ec7567db
(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
8b3b295502 (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>
2007-08-30 21:54:31 -07:00
..
calibrate.c [PATCH] remove many unneeded #includes of sched.h 2007-02-14 08:09:54 -08:00
do_mounts_initrd.c Freezer: make kernel threads nonfreezable by default 2007-07-17 10:23:02 -07:00
do_mounts_md.c
do_mounts_rd.c
do_mounts.c init: wait for asynchronously scanned block devices 2007-07-16 09:05:45 -07:00
do_mounts.h
initramfs.c initramfs: missing __init 2007-07-26 11:11:56 -07:00
Kconfig Kconfig: remove top level menu "Code maturity level options" 2007-07-31 15:39:43 -07:00
main.c fix maxcpus=1 oops in show_stat() 2007-08-30 21:54:31 -07:00
Makefile [PATCH] disable init/initramfs.c 2007-02-11 10:51:25 -08:00
noinitramfs.c [PATCH] disable init/initramfs.c 2007-02-11 10:51:25 -08:00
version.c [PATCH] remove many unneeded #includes of sched.h 2007-02-14 08:09:54 -08:00