Messages in this thread | | | Date | Thu, 25 Mar 2010 13:59:35 +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 2010-3-25 18:14, Oleg Nesterov wrote: > > On 03/25, Miao Xie wrote: > >> > >> 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 First of let me note that it is wrong to call scan_for_empty_cpusets() at CPU_DOWN_PREPARE state. _cpu_down() can fail after that but we can't revert the result of remove_tasks_in_empty_cpuset().
But this doesn't matter,
> and task->cpus_allowed, and then moves the task running on the dying cpu > to the other online cpu.
No, it doesn't track task->cpus_allowed afaics. It only checks cpumask_empty(cp->cpus_allowed) and does nothing otherwise.
And it is quite possible that the task belongs to some cpuset cs, bound to a single cpu, but cs->cpus_allowed is "wide" and includes other online cpus.
> At that time, rt-threads for each cpu have not > been created.
(doesn't matter, but the are already created and sleeping)
> And when the kernel does migration_call(CPU_DEAD), the rt-threads already > exit.
No, there are sleeping, but this doesn't matter again.
> the task that holds callback_mutex can run as normal.
It can't afaics, please see above.
That said, let me remind. I read this code only once a long ago, during my first attempt to fix these problems (all my attempts were ignored until I rerouted my concerns to Peter). It is possible that I missed/forgot/both something. But when I did the second version I bothered to actually test my theory and the kernel hanged, see the changelog in http://marc.info/?t=124910242400002
You was cc'ed too ;)
Oleg.
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