Re: b1cbacc866 ("x86/smpboot: Do not use smp_num_siblings in .."): divide error: 0000 [#1] SMP DEBUG_PAGEALLOC

[Prarit Bhargava]

>[    0.029102] Hierarchical SRCU implementation.
>[    0.030040] smp: Bringing up secondary CPUs ...
>[    0.030528] smp: Brought up 1 node, 1 CPU
>[    0.030953] divide error: 0000 [#1] SMP DEBUG_PAGEALLOC
>[    0.031000] Modules linked in:
>[    0.031000] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.14.0-01223-gb1cbacc #1
>[    0.031000] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
>[    0.031000] task: ffff88001e460000 task.stack: ffffc90000008000

A single socket/package, single core, single thread system
has 1 thread.  __max_smt_threads is not set to 1 since smp_callin() is never
called.  The minimum value of __max_smt_threads must be 1.

tglx, this is built on -tip.

P.
----8<----

Subject: [PATCH] arch/x86: Initialize __max_smt_threads to 1

A single socket, single core, single thread system has __max_smt_threads
set to 0 since smp_callin() is not called.

__max_smt_threads must be a minimum of 1.

Fixes: b1cbacc8663a ("x86/smpboot: Do not use smp_num_siblings in __max_logical_packages calculation)
Signed-off-by: Prarit Bhargava <prarit@xxxxxxxxxx>
Cc: Jakub Kicinski <kubakici@xxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Cc: "netdev@xxxxxxxxxxxxxxx"
Cc: Clark Williams <williams@xxxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Link: https://marc.info/?t=151246092100004&r=1&w=2
---
 arch/x86/kernel/smpboot.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index eaee15fb7d8b..882c61e1d7a2 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -106,7 +106,7 @@ EXPORT_SYMBOL(__max_logical_packages);
 static unsigned int logical_packages __read_mostly;
 
 /* Maximum number of SMT threads on any online core */
-int __max_smt_threads __read_mostly;
+int __read_mostly __max_smt_threads = 1;
 
 /* Flag to indicate if a complete sched domain rebuild is required */
 bool x86_topology_update;
-- 
2.15.0.rc0.39.g2f0e14e64