Messages in this thread |  | | | Date | Thu, 25 Mar 2010 11:14:20 +0100 | | From | Oleg Nesterov <> | | Subject | Re: [PATCH 1/6] kill the broken and deadlockable cpuset_lock/cpuset_cpus_allowed_locked code |
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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?
Oleg.
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