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    SubjectRe: [PATCH v8 3/3] cgroups: make procs file writable
    On Wed, Mar 9, 2011 at 10:18 PM, Ben Blum <bblum@andrew.cmu.edu> wrote:
    >> This BUG_ON() seems unnecessary, given the i++ directly above it.
    >
    > It's meant to communicate that the loop must go through at least once,
    > so that 'struct cgroup *oldcgrp' will be initialised within a loop later
    > (setting it to NULL in the beginning is just to shut up the compiler.)

    Right, but it's a do {} while() loop with no break in it - it's
    impossible to not go through at least once...

    >> Should we be setting failed_ss here? Doesn't that mean that if all
    >> subsystems pass the can_attach() check but the first one fails a
    >> can_attach_task() check, we don't call any cancel_attach() methods?
    >>
    >> What are the rollback semantics for failing a can_attach_task() check?
    >
    > They are not called in that order - it's for_each_subsys { can_attach();
    > can_attach_task(); }.

    Oh, fair point - I misread that.

    > Although if the deal is that cancel_attach reverts
    > the things that can_attach does (and can_attach_task is separate) (is
    > this the case? it should probably go in the documentation), then passing
    > a can_attach and failing a can_attach_task should cause cancel_attach to
    > get called for that subsystem, which in this code it doesn't. Something
    > like:
    >
    >    retval = ss->can_attach();
    >    if (retval) {
    >        failed_ss = ss;
    >        goto out_cancel_attach;
    >    }
    >    retval = ss->can_attach_task();
    >    if (retval) {
    >        failed_ss = ss;
    >        cancel_extra_ss = true;
    >        goto out_cancel_attach;
    >    }

    Yes, but maybe call the flag cancel_failed_ss? Slightly more obvious,
    to me at least.

    >> > + ? ? ? ? ? ? ? ? ? ? ? BUG_ON(!thread_group_leader(tsk));
    >>
    >> Can this race with an exiting/execing group leader?
    >
    > No, rcu_read_lock() is held.
    >

    But rcu_read_lock() doesn't stop any actions - it just stops the data
    structures from going away. Can't leadership change during an
    execve()?

    > (However, I did try to test it, and it looks like if a leader calls
    > sys_exit() then the whole group goes away; is this actually guaranteed?)

    I think so, but maybe not instantaneously.

    Paul
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