Messages in this thread |  | | Date | Thu, 17 Jan 2013 15:50:29 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] printk: Avoid softlockups in console_unlock() |
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On Fri, 18 Jan 2013 00:46:14 +0100 Jan Kara <jack@suse.cz> wrote:
> On Thu 17-01-13 13:39:17, Andrew Morton wrote: > > On Thu, 17 Jan 2013 22:04:42 +0100 > > Jan Kara <jack@suse.cz> wrote: > > > > > ... > > > > > > So I played a bit with this. To make things easier for me I added > > > artificial mdelay(len*10) (effectively simulating console able to print 100 > > > characters per second) just after call_console_drivers() so that I can > > > trigger issues even on a machine easily available to me. Booting actually > > > doesn't trigger any problems because there aren't enough things happening > > > in parallel on common machine during boot but > > > echo t >/proc/sysrq-trigger & > > > for i in /lib/modules/3.8.0-rc3-0-default/kernel/fs/*/*.ko; do > > > name=`basename $i`; name=${name%.ko}; modprobe $name > > > done > > > easily triggers the problem (as modprobe uses both RCU & IPIs to signal all > > > CPUs). > > > > > > Adding > > > touch_nmi_watchdog(); > > > touch_all_softlockup_watchdogs(); > > > rcu_cpu_stall_reset(); > > > > I'm not sure that touch_all_softlockup_watchdogs() is needed? > > touch_nmi_watchdog() itself calls touch_softlockup_watchdog(). > It is. I've tried without it and the machine died a horrible death > because softlockup reports added to already too heavy printk traffic. The > problem is that CPU doing printing cannot handle IPIs thus if someone calls > e.g. smp_call_function_many() that function will spin waiting for IPIs on > all CPUs to finish. And that doesn't happen until printing is done so > CPU doing smp_call_function_many() gets locked up as well.
erk. I trust we'll have a nice comment explaining this mechanism ;)
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