Messages in this thread | | | Date | Fri, 8 Aug 2008 13:30:49 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] printk: robustify printk |
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On Fri, Aug 08, 2008 at 12:14:28PM -0700, Andrew Morton wrote: > On Fri, 08 Aug 2008 20:14:28 +0200 > Peter Zijlstra <a.p.zijlstra@chello.nl> wrote: > > > void wake_up_klogd(void) > > { > > - if (!oops_in_progress && waitqueue_active(&log_wait)) > > - wake_up_interruptible(&log_wait); > > + unsigned long flags; > > + struct klogd_wakeup_state *kws; > > + > > + if (!waitqueue_active(&log_wait)) > > + return; > > + > > + local_irq_save(flags); > > + kws = &__get_cpu_var(kws); > > + if (!kws->pending) { > > + kws->pending = 1; > > + call_rcu(&kws->head, __wake_up_klogd); > > + } > > + local_irq_restore(flags); > > } > > Note that kernel/rcupreempt.c's flavour of call_rcu() takes > RCU_DATA_ME().lock, so there are still code sites from which a printk > can deadlock. Only now, it is config-dependent.
The RCU callbacks are (and must be) invoked without holding any RCU locks, so printk()s in RCU callbacks are perfectly permissible. It would not be -that- hard to eliminate the lock in call_rcu(), but yeccch... It would be far easier to debug RCU without the benefit printk()s than to deal with a preemptable RCU implementation that was based solely on atomic instructions!!!
> >From a quick look it appears that large amounts of kernel/rcupreempt.c > are now a printk-free zone.
The only printk() is in preemptable RCU's rcu_init() function, whose only purpose is to mark the dmesg appropriately, and which should not be a problem.
And that would be because I do a fair amount of prototyping and debugging in user space, where one instead uses printf(). ;-)
Thanx, Paul
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