Re: [PATCH] MIPS: clear idle task mm pointer when hotplugging cpu

[Ralf Baechle <ralf@linux-mips.org>]

On Tue, Feb 08, 2011 at 04:18:07PM -0800, maksim.rayskiy@gmail.com wrote:

> If kernel starts with maxcpus= option which does not bring all
> available cpus online at boot time, idle tasks for offline cpus
> are not created. If later offline cpus are hotplugged through sysfs,
> __cpu_up is called in the context of the user task, and fork_idle
> copies its non-zero mm pointer.  This causes BUG() in per_cpu_trap_init.
> 
> To avoid this, release mm for idle task and reset the pointer after
> fork_idle().

Nice catch, applied.

x86 avoid this problem by forking the idle threads in a worker thread which
also avoids other potencial issues; we probably should take the same
path.

  Ralf