[PATCH] timer: Fix unsafe cpu variable access in migrate_timers

[Jan Kiszka <jan.kiszka@web.de>]

From: Jan Kiszka <jan.kiszka@siemens.com>

migrate_timers is invoked with preemption enabled. Therefore we have to
get/put the cpu-local variable tvec_bases like before commit 0eeda71bc3.

This fixes

BUG: using smp_processor_id() in preemptible [00000000] code: bash/4917
caller is debug_smp_processor_id+0x17/0x19
CPU: 0 PID: 4917 Comm: bash Not tainted 4.1.0-dbg+ #97
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.8.1-0-g4adadbd-20150316_085822-nilsson.home.kraxel.org 04/01/2014
 ffff880038674040 ffff880002e6fb98 ffffffff815356a0 0000000000000002
 0000000000000000 ffff880002e6fbc8 ffffffff8130c16f ffff88003fd8d500
 000000000000d500 0000000000000000 0000000000000003 ffff880002e6fbd8
Call Trace:
 [<ffffffff815356a0>] dump_stack+0x4f/0x7b
 [<ffffffff8130c16f>] check_preemption_disabled+0xdd/0xef
 [<ffffffff8130c198>] debug_smp_processor_id+0x17/0x19
 [<ffffffff810a9b37>] timer_cpu_notify+0x4f/0x112
 [<ffffffff8106718b>] notifier_call_chain+0x4c/0x71
 [<ffffffff810671be>] __raw_notifier_call_chain+0xe/0x10
 [<ffffffff810484b6>] __cpu_notify+0x20/0x37
 [<ffffffff810484e0>] cpu_notify+0x13/0x15
 [<ffffffff81048591>] cpu_notify_nofail+0xe/0x16
 [<ffffffff8152efb2>] _cpu_down+0x178/0x268
 [<ffffffff8108bc3a>] ? trace_hardirqs_on+0xd/0xf
 [<ffffffff8152f0ca>] cpu_down+0x28/0x3c
 [<ffffffff813cbdb9>] cpu_subsys_offline+0x14/0x16
 [<ffffffff813c779d>] device_offline+0x7d/0xb1
 [<ffffffff813c78a2>] online_store+0x48/0x68
 [<ffffffff813c5544>] dev_attr_store+0x18/0x22
 [<ffffffff811dac6c>] sysfs_kf_write+0x49/0x51
 [<ffffffff811da139>] kernfs_fop_write+0x105/0x158
 [<ffffffff8116c54f>] __vfs_write+0x28/0xbd
 [<ffffffff812ab014>] ? security_file_permission+0x23/0x90
 [<ffffffff8116ccb2>] vfs_write+0xb2/0x169
 [<ffffffff8116d717>] SyS_write+0x4a/0x91
 [<ffffffff8153d12e>] entry_SYSCALL_64_fastpath+0x12/0x76

triggered when offlining a CPU, e.g. via sysfs.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---
 kernel/time/timer.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 520499d..c826178 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1566,7 +1566,7 @@ static void migrate_timers(int cpu)
 
 	BUG_ON(cpu_online(cpu));
 	old_base = per_cpu_ptr(&tvec_bases, cpu);
-	new_base = this_cpu_ptr(&tvec_bases);
+	new_base = &get_cpu_var(tvec_bases);
 	/*
 	 * The caller is globally serialized and nobody else
 	 * takes two locks at once, deadlock is not possible.
@@ -1590,6 +1590,7 @@ static void migrate_timers(int cpu)
 
 	spin_unlock(&old_base->lock);
 	spin_unlock_irq(&new_base->lock);
+	put_cpu_var(tvec_bases);
 }
 
 static int timer_cpu_notify(struct notifier_block *self,