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    SubjectRe: [PATCH 0/3] resend, cpuset/hotplug fixes
    On 09/11, Lai Jiangshan wrote:
    > I have different concept. cpuset_cpus_allowed() is not called at atomic
    > context nor non-preemptable context nor other critical context.
    > So it should be allowed to use mutexs. That's what I think.

    Well, it is called from non-preemptable context: move_task_off_dead_cpu().
    That is why before this patch we had cpuset_cpus_allowed_lock(). And this
    imho adds unneeded complications.

    And I can't understand why sched_setaffinity() path should take the
    global mutex instead of per-cpuset spinlock.

    > There is a bug when migration_call() requires a mutex
    > before migration has been finished when cpu offline as Oleg described.
    > Bug this bug is only happened when cpu offline. cpu offline is rare and
    > is slowpath. I think we should fix cpu offline and ensure it requires
    > the mutex safely.

    This is subjective, but I can't agree. I think we should fix cpusets
    instead. We should try to avoid the dependencies between different
    subsystems as much as possible.

    > Oleg's patch moves all dirty things into CPUSET subsystem and makes
    > cpuset_cpus_allowed() does not require any mutex and increases CPUSET's
    > coupling. I don't feel it's good.

    Again, subjective... But I can't understand "increases CPUSET's coupling".

    From my pov, this patch cleanups and simplifies the code. This was the
    main motivation, the bugfix is just the "side effect". I don't understand
    how this subtle cpuset_lock() can make things better. I can't understand
    why we need the global lock to calc cpus_allowed.

    > > > cpuset_cpus_allowed() is not only used for CPU offline.
    > > > >
    > > > > sched_setaffinity() also uses it.
    > >
    > > Sure. And it must take get_online_cpus() to avoid the races with hotplug.
    > Oleg hasn't answered that
    > "is it safe when pdflush() calls cpuset_cpus_allowed()?".

    Because I didn't see such a question ;) perhaps I missed it previously...

    > A patch may be needed to ensure pdflush() calls cpuset_cpus_allowed() safely.

    What is wrong with pdflush()->cpuset_cpus_allowed() ? Why this is not safe?


    cpuset_cpus_allowed(current, cpus_allowed);
    set_cpus_allowed_ptr(current, cpus_allowed);

    looks equally racy, with or without the patch. But this is a bit off-topic,
    mm/pdflush.c has gone away.

    > One other minor thing:
    > Oleg's patch may introduce a trouble in PREEEMPT_RT tree, because
    > spinlock in RT is also mutex. Likely I'm wrong.

    Yes, probably -rt need raw_lock (as you pointed out).


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