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

[Miao Xie]

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.

Thanks!
Miao

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