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    SubjectRe: [PATCH v7 2/3] cgroups: add atomic-context per-thread subsystem callbacks
    On Mon, Jan 24, 2011 at 12:38:06AM -0800, Paul Menage wrote:
    > Hi Ben,
    >
    > Finally finding a moment to actually look at these patches. Sorry it's
    > been a while. Can you send the patches inline rather than as
    > attachments in future?

    Whoops, sure thing.

    >
    > Reviewed-by: Paul Menage <menage@google.com>
    >
    > This patch looks fine, although I think that freezer_can_attach_task()
    > could be simplified to:
    >
    > static int freezer_can_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
    > {
    > if (__cgroup_freezing_or_frozen(tsk))
    > return -EBUSY;
    > return 0;
    > }
    >
    > since we guarantee that rcu_read_lock() is held across this call.

    I put a note there that "rcu_read_lock allows recursive locking", to
    denote that it's okay to double-lock when it's called from
    cgroup_attach_proc. I guess this isn't very clear: the reason the lock
    is there is because in cgroup_attach_task, I call it without
    rcu_read_lock (not necessary in most cases), but freezer needs RCU there
    in either case. I wrote in the documentation: "This may run in
    rcu_read-side", which I guess isn't very clear either.

    >
    > There appears to be a tiny bit of rot in kernel/cpu.c (due to the
    > addition of the exit() callback) and memcontrol.c (due to some changes
    > at the start of mem_cgroup_move_task()) but neither impact actual
    > code.
    >
    > I think that before actually pushing to mainline, we'll need to sort
    > out the cpuset mempolicy yielding issue, since that could be a
    > user-visible API change.
    >
    >
    > Paul

    Hmm. The quirks caused by this are specific to using cgroup.procs, and
    since cgroup.procs is new, I wouldn't say this is an API "change"?

    Thanks,
    Ben


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