linux-kernel - [tip:core/urgent] smpboot: Add missing get_online_cpus() in smpboot_register_percpu

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Message-ID: <tip-4bee96860a65c3a62d332edac331b3cf936ba3ad@git.kernel.org>
Date:	Fri, 23 Jan 2015 02:36:29 -0800
From:	tip-bot for Lai Jiangshan <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	srivatsa.bhat@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	laijs@...fujitsu.com, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org,
	peterz@...radead.org, rientjes@...gle.com
Subject: [tip:core/urgent] smpboot: Add missing get_online_cpus()
  in smpboot_register_percpu_thread()

Commit-ID:  4bee96860a65c3a62d332edac331b3cf936ba3ad
Gitweb:     http://git.kernel.org/tip/4bee96860a65c3a62d332edac331b3cf936ba3ad
Author:     Lai Jiangshan <laijs@...fujitsu.com>
AuthorDate: Thu, 31 Jul 2014 11:30:17 +0800
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Fri, 23 Jan 2015 11:33:51 +0100

smpboot: Add missing get_online_cpus() in smpboot_register_percpu_thread()

The following race exists in the smpboot percpu threads management:

CPU0	      	   	     CPU1
cpu_up(2)
  get_online_cpus();
  smpboot_create_threads(2);
			     smpboot_register_percpu_thread();
			     for_each_online_cpu();
			       __smpboot_create_thread();
  __cpu_up(2);

This results in a missing per cpu thread for the newly onlined cpu2 and
in a NULL pointer dereference on a consecutive offline of that cpu.

Proctect smpboot_register_percpu_thread() with get_online_cpus() to
prevent that.

[ tglx: Massaged changelog and removed the change in
        smpboot_unregister_percpu_thread() because that's an
        optimization and therefor not stable material. ]

Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Rusty Russell <rusty@...tcorp.com.au>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
Cc: David Rientjes <rientjes@...gle.com>
Cc: stable@...r.kernel.org
Link: http://lkml.kernel.org/r/1406777421-12830-1-git-send-email-laijs@cn.fujitsu.com
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/smpboot.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index f032fb5..40190f2 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -280,6 +280,7 @@ int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread)
 	unsigned int cpu;
 	int ret = 0;
 
+	get_online_cpus();
 	mutex_lock(&smpboot_threads_lock);
 	for_each_online_cpu(cpu) {
 		ret = __smpboot_create_thread(plug_thread, cpu);
@@ -292,6 +293,7 @@ int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread)
 	list_add(&plug_thread->list, &hotplug_threads);
 out:
 	mutex_unlock(&smpboot_threads_lock);
+	put_online_cpus();
 	return ret;
 }
 EXPORT_SYMBOL_GPL(smpboot_register_percpu_thread);
--
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