Messages in this thread | | | Subject | Re: [PATCH] printk: robustify printk | From | Peter Zijlstra <> | Date | Fri, 08 Aug 2008 21:49:27 +0200 |
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On Fri, 2008-08-08 at 12:37 -0700, Andrew Morton wrote: > On Fri, 08 Aug 2008 21:21:08 +0200 > Peter Zijlstra <a.p.zijlstra@chello.nl> wrote: > > > On Fri, 2008-08-08 at 12:14 -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. > > > > > > From a quick look it appears that large amounts of kernel/rcupreempt.c > > > are now a printk-free zone. > > > > Drad, missed that bit, I did look at the calling end, but forgot the > > call_rcu() end :-/ > > > > The initial printk_tick() based implementation didn't suffer this > > problem, should we revert to that scheme? > > Dunno. Perhaps we could convert RCU_DATA_ME's spinlock_t into an > rwlock and do read_lock() in call_rcu()? Then we can should be able to > call printk from inside that read_lock(), but not inside write_lock(), > which, with suitable warning comments might be acceptable. > > afacit everything in call_rcu()'s *rdp is cpu-local and is protected by > local_irq_save(). rcu_ctrlblk.completed and rcu_flipped need some > protection, but a) rdp->lock isn't sufficient anyway and b) > read_lock protection would suffice. Maybe other CPUs can alter *rdp > while __rcu_advance_callbacks() is running. > > Anyway, that's all handwaving. My point is that making rcupreempt.c > more robust and more concurrent might be an alternative fix, and might > be beneficial in its own right. Working out the details is what we > have Pauls for ;)
Hehe :-)
Just in case Paul shows he's human and cannot work his way around it, I just posted a new version of the printk_tick() stuff..
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