Re: [PATCH 6/6] kernel: Avoid softlockups in stop_machine() during heavy printing

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 13 Mar 2014 16:58:38 +0100 Jan Kara <jack@suse.cz> wrote:

> When there are lots of messages accumulated in printk buffer, printing
> them (especially over serial console) can take a long time (tens of
> seconds). stop_machine() will effectively make all cpus spin in
> multi_cpu_stop() waiting for the CPU doing printing to print all the
> messages which triggers NMI softlockup watchdog and RCU stall detector
> which add even more to the messages to print. Since machine doesn't do
> anything (except serving interrupts) during this time, also network
> connections are dropped and other disturbances may happen.
> 
> Paper over the problem by waiting for printk buffer to be empty before
> starting to stop CPUs. In theory a burst of new messages can be appended
> to the printk buffer before CPUs enter multi_cpu_stop() so this isn't a 100%
> solution but it works OK in practice and I'm not aware of a reasonably
> simple better solution.
> 

Yes it's rather hacky, but it's simple and direct and explicit and
obvious.  It's the stealth hackiness which causes harm.