[PATCH] x86: check for online cpus before bringing them up

Bryce reported a bug wherein offlining CPU0 (on x86 box) and then
subsequently onlining it resulted in a lockup.

On x86, CPU0 is never offlined.  The subsequent attempt to online CPU0
doesn't take that into account.  It actually tries to bootup the already
booted CPU.  Following patch fixes the problem (as acknowledged by Bryce).
Please consider for inclusion in 2.6.16.

Check if cpu is already online.

Signed-off-by: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 10 insertions, 0 deletions

diff --git a/arch/i386/kernel/smpboot.c b/arch/i386/kernel/smpboot.c
index eba7f53f8b4a..7007e1783797 100644
--- a/arch/i386/kernel/smpboot.c
+++ b/arch/i386/kernel/smpboot.c
@@ -1029,6 +1029,16 @@ int __devinit smp_prepare_cpu(int cpu)
         int     apicid, ret;
 
         lock_cpu_hotplug();
+
+        /*
+         * On x86, CPU0 is never offlined.  Trying to bring up an
+         * already-booted CPU will hang.  So check for that case.
+         */
+        if (cpu_online(cpu)) {
+                ret = -EINVAL;
+                goto exit;
+        }
+
         apicid = x86_cpu_to_apicid[cpu];
         if (apicid == BAD_APICID) {
                 ret = -ENODEV;