CC'd Gautham R Shenoy <ego@in.ibm.com>.

I believe that we had the locking relation between what had been
cgroup_lock (global cgroup lock which can be held over large stretches
of non-performance critical code) and callback_mutex (global cpuset
specific lock which is held over shorter stretches of more performance
critical code - though still not on really hot code paths.)  One can
nest callback_mutex inside cgroup_lock, but not vice versa.

The callback_mutex guarded some CPU masks and Node masks, which might
be multi-word and hence don't change atomically.  Any low level code
that needs to read these these cpuset CPU and Node masks, needs to
hold callback_mutex briefly, to keep that mask from changing while
being read.

There is even a comment in kernel/cpuset.c, explaining how an ABBA
deadlock must be avoided when calling rebuild_sched_domains():

/*
 * rebuild_sched_domains()
 *
 * ...
 *
 * Call with cgroup_mutex held.  May take callback_mutex during
 * call due to the kfifo_alloc() and kmalloc() calls.  May nest
 * a call to the get_online_cpus()/put_online_cpus() pair.
 * Must not be called holding callback_mutex, because we must not
 * call get_online_cpus() while holding callback_mutex.  Elsewhere
 * the kernel nests callback_mutex inside get_online_cpus() calls.
 * So the reverse nesting would risk an ABBA deadlock.

This went into the kernel sometime around 2.6.18.

Then in October and November of 2007, Gautham R Shenoy submitted
"Refcount Based Cpu Hotplug" (http://lkml.org/lkml/2007/11/15/239)

This added cpu_hotplug.lock, which at first glance seems to fit into
the locking hierarchy about where callback_mutex did before, such as
being invocable from rebuild_sched_domains().

However ... the kernel/cpuset.c comments were not updated to describe
the intended locking hierarchy as it relates to cpu_hotplug.lock, and
it looks as if cpu_hotplug.lock can also be taken while invoking the
hotplug callbacks, such as the one here that is handling a CPU down
event for cpusets.

Gautham ... you there?

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@sgi.com> 1.940.382.4214
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