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    SubjectRe: [PATCH][BUGFIX] cgroups: more safe tasklist locking in cgroup_attach_proc
    On Fri, Jul 29, 2011 at 7:28 AM, Ben Blum <bblum@andrew.cmu.edu> wrote:
    > Fix unstable tasklist locking in cgroup_attach_proc.
    >
    > From: Ben Blum <bblum@andrew.cmu.edu>
    >
    > According to this thread - https://lkml.org/lkml/2011/7/27/243 - RCU is
    > not sufficient to guarantee the tasklist is stable w.r.t. de_thread and
    > exit. Taking tasklist_lock for reading, instead of rcu_read_lock,
    > ensures proper exclusion.
    >
    > Signed-off-by: Ben Blum <bblum@andrew.cmu.edu>

    Acked-by: Paul Menage <menage@google.com>

    Thanks,
    Paul

    > ---
    >  kernel/cgroup.c |    6 +++---
    >  1 file changed, 3 insertions(+), 3 deletions(-)
    >
    > diff a/kernel/cgroup.c b/kernel/cgroup.c
    > --- a/kernel/cgroup.c   2011-07-21 19:17:23.000000000 -0700
    > +++ b/kernel/cgroup.c   2011-07-29 06:17:47.000000000 -0700
    > @@ -2024,7 +2024,7 @@
    >                goto out_free_group_list;
    >
    >        /* prevent changes to the threadgroup list while we take a snapshot. */
    > -       rcu_read_lock();
    > +       read_lock(&tasklist_lock);
    >        if (!thread_group_leader(leader)) {
    >                /*
    >                 * a race with de_thread from another thread's exec() may strip
    > @@ -2033,7 +2033,7 @@
    >                 * throw this task away and try again (from cgroup_procs_write);
    >                 * this is "double-double-toil-and-trouble-check locking".
    >                 */
    > -               rcu_read_unlock();
    > +               read_unlock(&tasklist_lock);
    >                retval = -EAGAIN;
    >                goto out_free_group_list;
    >        }
    > @@ -2054,7 +2054,7 @@
    >        } while_each_thread(leader, tsk);
    >        /* remember the number of threads in the array for later. */
    >        group_size = i;
    > -       rcu_read_unlock();
    > +       read_unlock(&tasklist_lock);
    >
    >        /*
    >         * step 1: check that we can legitimately attach to the cgroup.
    >
    --
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