Re: [Lse-tech] [PATCH] cpusets+hotplug+preepmt broken

[Nathan Lynch <ntl@pobox.com>]

On Wed, May 11, 2005 at 12:55:08PM -0700, Paul Jackson wrote:
> pj wrote:
> > Could you explain why this is -- what is the deadlock?
> 
> On further reading, I see it.  You're right.
> 
> Deep in the bowels of the hotplug code, when removing a dead cpu, while
> holding the runqueue lock (task_rq_lock), the hotplug code might need to
> walk up the cpuset hierarchy, to find the nearest enclosing cpuset that
> still has online cpus, as part of figuring where to run a task that is
> being kicked off the dead cpu.  The runqueue lock is atomic, but getting
> the cpuset_sem (needed to walk up the cpuset hierarchy) can sleep.  So
> you need to get the cpuset_sem before calling task_rq_lock, so as to
> avoid the "scheduling while atomic" oops that you reported.  Therefore
> the hotplug code, and anyone else calling cpuset_cpus_allowed(), which
> means sched_setaffinity(), needs to first grab cpuset_sem, so that they
> can grab any atomic locks needed, after getting cpuset_sem, not before.

Won't holding cpuset_sem while calling cpuset_cpus_allowed cause a
deadlock?

Nathan