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    SubjectRe: [PATCH] move put_task_struct() reaping into a thread [Re: 2.6.18-rt1]
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    Bill Huey (hui) <billh@gnuppy.monkey.org> writes:

    > On Tue, Sep 26, 2006 at 08:55:41PM -0600, Eric W. Biederman wrote:
    >> Bill Huey (hui) <billh@gnuppy.monkey.org> writes:
    >> > This patch moves put_task_struct() reaping into a thread instead of an
    >> > RCU callback function as discussed with Esben publically and Ingo privately:
    >>
    >> Stupid question.
    >
    > It's a great question actually.
    >
    >> Why does the rt tree make all calls to put_task_struct an rcu action?
    >> We only need the rcu callback from kernel/exit.c
    >
    > Because the conversion of memory allocation routines like kmalloc and kfree aren't
    > safely callable within a preempt_disable critical section since they were incompletely
    > converted in the -rt. It can run into the sleeping in atomic scenario which can result
    > in a deadlock since those routines use blocking locks internally in the implementation
    > now as a result of the spinlock_t conversion to blocking locks.

    Interesting. I think the easy solution would just be to assert that put_task_struct
    can sleep and to fix any callers that expect differently. I haven't looked very
    closely but I don't recall anything that needs put_task_struct to be atomic.
    With a function that complex I certainly would not expect it to never sleep unless
    it had a big fat comment.

    Well I did find an instance where we call put_task_struct with a
    spinlock held. Inside of lib/rwsem.c:rwsem_down_failed_common().

    Still that may be the only user that cares. I suspect with a little
    code rearrangement that case is fixable. It's not like that code is a
    fast path or anything. It should just be a matter of passing the
    task struct outside of the spinlock before calling put_task_struct.

    >> Nothing else needs those semantics.
    >
    > Right, blame it on the incomplete conversion of the kmalloc and friends. GFP_ATOMIC is
    > is kind of meaningless in the -rt tree and it might be a good thing to add something
    > like GFP_RT_ATOMIC for cases like this to be handled properly and restore that
    > particular semantic in a more meaningful way.

    But this is a path where we are freeing data, so GFP_ATOMIC should not come
    into it. I just read through the code and there are not allocations
    there.

    >> I agree that put_task_struct is the most common point so this is unlikely
    >> to remove your issues with rcu callbacks but it just seems completely backwards
    >> to increase the number of rcu callbacks in the rt tree.
    >
    > I'm not sure what mean here, but if you mean that you don't like the RCU API abuse then
    > I agree with you on that. However, Ingo disagrees and I'm not going to argue it with him.
    > Although, I'm not going stop you if you do. :)

    What I was thinking is that rcu isn't terribly friendly to realtime
    activities because it postpones work and can wind up with a lot of
    work to do at some random time later which can be bad for latencies.

    So I was very surprised to see an rt patch making more things under
    rcu protection. Especially as I have heard other developers worried
    about rt issues discussing removing the rcu functionality.

    My gut feel now that I understand the pieces is that this approach has
    all of the hallmarks of a hack, both the kmalloc/kfree thing and even
    more calling put_task_struct in an atomic context. If the callers
    were fixed put_task_struct could safely sleep so kmalloc/kfree
    sleeping would not be a problem.

    Eric

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