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

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Date:   Tue,  5 Dec 2017 09:30:36 -0500
From:   Prarit Bhargava <prarit@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Prarit Bhargava <prarit@...hat.com>,
        Jakub Kicinski <kubakici@...pl>, netdev@...r.kernel.org,
        Clark Williams <williams@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>, x86@...nel.org
Subject: Re: b1cbacc866 ("x86/smpboot: Do not use smp_num_siblings in .."):  divide error: 0000 [#1] SMP DEBUG_PAGEALLOC

>[    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@...hat.com>
Cc: Jakub Kicinski <kubakici@...pl>
Cc: netdev@...r.kernel.org
Cc: "netdev@...r.kernel.org"
Cc: Clark Williams <williams@...hat.com>
Cc: Andi Kleen <ak@...ux.intel.com>
Cc: x86@...nel.org
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