On 2015年08月12日 20:31, Peter Hurley wrote: > On 08/11/2015 02:16 PM, Greg Kroah-Hartman wrote: >> On Tue, Aug 11, 2015 at 07:23:01PM +0800, Pan Xinhui wrote: >>> From: Pan Xinhui <xinhuix....@intel.com> >>> >>> printk can be called in any context, It's very useful to output debug >>> info. >>> >>> But it might cause very bad issues on some special cases. For example, >>> some driver hit errors, and it dumps many messages like reg values, etc. >>> >>> Sometimes, printk is called when irqs disabled. This is OKay if there is >>> a few messages. But What would happen if many messages outputted by other >>> drivers at same time. >>> >>> Here is the scenario. >>> CPUA CPUB >>> local_irq_save(flags); >>> printk() >>> while(..) { --> console_unlock >>> printk(...); >>> //hundreds or thousands loops >>> } //all messages flushed out to consoles >>> local_irq_restore(flags); >>> >> >> Where are you seeing this type of scenario "in the wild"? Or is this >> just a "debug/bringup hardware" issue? > > There have been problem reports of big machines getting soft-lockup/RCU stall > warnings with serial console attached. I think SLES is carrying patches > from Jan Kara to try to workaround this issue. > > commit 5874af2003b1aaaa053128d655710140e3187226 ("printk: enable interrupts before calling console_trylock_for_printk()") does help much in most cases. Thanks Jan Kara for that patch!
However there are still some corner cases like that I mentioned above. As preempt is disabled in console_unlock(), Even applying my patch, softlockup/RCU warning still goes out. I am very looking forward to Jan Kara's new patch to walk around it. Of course I will also try to work out a pre-review patch with my new idea to fix these softlockpup/RCU warning. Thanks xinhui >> We shouldn't be ever stuck in a >> printk that prints hundreds or thousands of loops, if so, we need to fix >> the kernel code that does that, as we do have control over this. > > The loop referred to here is the loop in console_unlock(). Essentially > what happens is one cpu can get trapped in the console_unlock() output > loop; printk()'s from other cpus are only appending to the logbuf since > they can't acquire the console_lock (which is owned by the one cpu trapped > in the output loop). > > Regards, > Peter Hurley > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/