From: Oleg Nesterov on
On 03/25, Miao Xie wrote:
> on 2010-3-15 17:10, Oleg Nesterov wrote:
> > This patch just states the fact the cpusets/cpuhotplug interaction is
> > broken and removes the deadlockable code which only pretends to work.
> >
> > - cpuset_lock() doesn't really work. It is needed for
> > cpuset_cpus_allowed_locked() but we can't take this lock in
> > try_to_wake_up()->select_fallback_rq() path.
> >
> > - cpuset_lock() is deadlockable. Suppose that a task T bound to CPU takes
> > callback_mutex. If cpu_down(CPU) happens before T drops callback_mutex
> > stop_machine() preempts T, then migration_call(CPU_DEAD) tries to take
> > cpuset_lock() and hangs forever because CPU is already dead and thus
> > T can't be scheduled.
> The problem what you said don't exist, because the kernel already move T to
> the active cpu when preparing to turn off a CPU.

we need cpuset_lock() to move T. please look at _cpu_down().


A task T holds callback_mutex, and it is bound to CPU 1.

_cpu_down(cpu => 1) is called by the task X.

_cpu_down()->stop_machine() spawns rt-threads for each cpu,
a thread running on CPU 1 preempts T and calls take_cpu_down()
which removes CPU 1 from online/active masks.

X continues, and does raw_notifier_call_chain(CPU_DEAD), this
calls migration_call(CPU_DEAD), and _this_ is what move the
tasks from the dead CPU.

migration_call(CPU_DEAD) calls cpuset_lock() and deadlocks.



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