printk: Fix wake_up_klogd() vs cpu hotplug

commit 49f4138346b3cec2706adff02658fe27ceb1e46f upstream.

wake_up_klogd() may get called from preemptible context but uses
__raw_get_cpu_var() to write to a per cpu variable. If it gets preempted
between getting the address and writing to it, the cpu in question could be
offline if the process gets scheduled back and hence writes to the per cpu data
of an offline cpu.

This buggy behaviour was introduced with fa33507a "printk: robustify
printk, fix #2" which was supposed to fix a "using smp_processor_id() in
preemptible" warning.

Let's use this_cpu_write() instead which disables preemption and makes sure
that the outlined scenario cannot happen.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20101126124247.GC7023@osiris.boeblingen.de.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/kernel/printk.c b/kernel/printk.c
index faa9f5915480ca45718be78cd910f73bd6567279..44a2aa76baa455cf09d2b75113227919a4e43181 100644 (file)
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -1070,7 +1070,7 @@ int printk_needs_cpu(int cpu)
 void wake_up_klogd(void)
 {
        if (waitqueue_active(&log_wait))
-               __raw_get_cpu_var(printk_pending) = 1;
+               this_cpu_write(printk_pending, 1);
 }
 
 /**