Re: [PATCH 1/6] kill the broken and deadlockable cpuset_lock/cpuset_cpus_allowed_locked code

[Miao Xie <miaox@cn.fujitsu.com>]

on 2010-3-25 18:14, Oleg Nesterov wrote:
> 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().
> 
> OK.
> 
> 	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.
> 
> See?

But when the kernel want to offline a cpu, it does 
	raw_notifier_call_chain(CPU_DOWN_PREPARE) 
at first. this calls cpuset_track_online_cpus() to update cpuset's cpus
and task->cpus_allowed, and then moves the task running on the dying cpu
to the other online cpu. At that time, rt-threads for each cpu have not
been created.

And when the kernel does migration_call(CPU_DEAD), the rt-threads already
exit. the task that holds callback_mutex can run as normal.

Miao

> 
> Oleg.
> 
> 
> 
>