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SubjectRe: [patch 1/3] printk: fix wake_up_klogd() vs cpu hotplug
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On Fri, 2010-11-26 at 13:00 +0100, Heiko Carstens wrote:
> plain text document attachment (001_printk_preempt.diff)
> From: Heiko Carstens <heiko.carstens@de.ibm.com>
>
> 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@de.ibm.com>
> ---
> 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);
> + }
> }
>
> /**
>

But but but, the cpu can still be offlined between writing this state
and the next tick happening, right?


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