Messages in this thread | | | Date | Fri, 26 Nov 2010 13:13:25 +0100 | From | Heiko Carstens <> | Subject | Re: [patch 1/3] printk: fix wake_up_klogd() vs cpu hotplug |
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On Fri, Nov 26, 2010 at 01:10:08PM +0100, Peter Zijlstra wrote: > 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?
Yes, that's what the second patch would fix as a side effect.
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