[patch 1/3] printk: fix wake_up_klogd() vs cpu hotplug

[Heiko Carstens]

From: Heiko Carstens <heiko.carstens@xxxxxxxxxx>

wake_up_klogd() may get called from preemtible 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.

No idea why that 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 get_cpu_var() instead which disables preemption and makes sure that
the outlined scenario cannot happen.

Signed-off-by: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
---
 kernel/printk.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -1087,8 +1087,10 @@ int printk_needs_cpu(int cpu)
 
 void wake_up_klogd(void)
 {
-	if (waitqueue_active(&log_wait))
-		__raw_get_cpu_var(printk_pending) = 1;
+	if (waitqueue_active(&log_wait)) {
+		get_cpu_var(printk_pending) = 1;
+		put_cpu_var(printk_pending);
+	}
 }
 
 /**

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