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Subject[RFC PATCH 0/4] Refcount Based Cpu-Hotplug Revisit.
This patch series attempts to revisit the topic of
cpu-hotplug locking model.

Prior to this attempt, there were several different suggestions on
how it should be implemented.

The ones that were posted before were

a) Refcount + Waitqueue model:
Here the threads that want to avoid a cpu hotplug operation while
they are operating in cpu-hotplug critical section, bump up the
reference to the global online cpu state.
The thread which wants to perform a cpu-hotplug,
blocks until the reference to the global online state goes
to zero. Any threads which want to enter the cpu-hotplug critical
section during an ongoing cpu-hotplug operatoin, are blocked using
a waitqueue.

The advantange of this model was that it is along the lines of
the well known get/put model. Only that it allows sleeping of readers
and writers.

The disadvantage, as Andrew pointed out was that there do exist
a whole bunch of lock_cpu_hotplug()'s whose existance is undocumented,
and an approach like this will not improve such a situation.

b) Per Subsystem cpu-hotplug locks: Each subsystem which has cpu-hotplug
critical data, uses a lock to protect that data. Such a subsystem
needs to subscribe to the cpu-hotplug notification, especially the
CPU_LOCK_ACQUIRE and CPU_LOCK_RELEASE events which are sent before
and and after a cpu-hotplug operation. While handling these events
respectively, the subsystem lock is taken or released.

The advantage this model offered was that lock was associated with the
data, which made easy to understand the purpose of locking.

The disadvantage was that any cpu-hotplug aware function, could
not be called from a cpu-hotplug callback path, since we would have
acquired the subsystem lock during CPU_LOCK_ACQUIRE and attempting
to reacquire it would result in a deadlock.
The case which pointed this limitation out was the implementation of
synchronize_sched in preemptible rcu.

c) Freezer based cpu-hotplug:
The idea here was to freeze the system using the process freezer
technique which is being used for suspend/hibernate purpose, before
performing a cpu-hotplug operation. This would ensure that none of
the kernel threads are accessing any of the cpu-hotplug critical
data, because they are frozen at well known points.

This would have helped to remove all kinds of locks because when a
thread is accessing a cpu-hotplug critical data, it meant that the
system was not frozen and hence there would be no cpu-hotplug
operation untill the thread either voluntarily calls try_to_freeze
or returns out of the kernel.

The disadvantage of this approach was that any kind of dependencies
between threads might call the freezer to fail. For eg, thread A is
waiting for thread B to accomplish something, but thread B is already
frozen, leading to a freeze failure. There could be other subtle
races which might be difficult to track.

Some time in May 2007, Linus suggested using the refcount model, and
this patch series simplifies and reimplements the Refcount + waitqueue
model, based on the discussions in the past and inputs from
Vatsa and Rusty.

Patch 1/4: Implements the core refcount + waitqueue model.
Patch 2/4: Replaces all the lock_cpu_hotplug/unlock_cpu_hotplug instances
with get_online_cpus()/put_online_cpus()
Patch 3/4: Replaces the subsystem mutexes (we do have three of them now,
in sched.c, slab.c and workqueue.c) with get_online_cpus,
Patch 4/4: Eliminates the CPU_DEAD and CPU_UP_CANCELLED event handling
from workqueue.c

The patch series has survived an overnight test with kernbench on i386.
and has been tested with Paul Mckenney's latest preemptible rcu code.

Awaiting thy feedback!

Thanks and Regards
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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