Messages in this thread | | | Subject | Re: [patch 1/3] printk: fix wake_up_klogd() vs cpu hotplug | From | Eric Dumazet <> | Date | Fri, 26 Nov 2010 14:01:52 +0100 |
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Le vendredi 26 novembre 2010 à 13:42 +0100, Heiko Carstens a écrit : > On Fri, Nov 26, 2010 at 01:35:09PM +0100, Eric Dumazet wrote: > > Le vendredi 26 novembre 2010 à 13:00 +0100, Heiko Carstens a écrit : > > > 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); > > > + } > > > } > > > > Please use : > > > > this_cpu_write(printk_pending, 1); > > > > It is faster on x86, and does the right thing too. > > Ah, right. I wasn't aware that such a thing even exists. > Updated patch below: > > Subject: [PATCH] printk: fix wake_up_klogd() vs cpu hotplug > > 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 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> > --- > kernel/printk.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > --- a/kernel/printk.c > +++ b/kernel/printk.c > @@ -1088,7 +1088,7 @@ 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); > } > > /**
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
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