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    Subject[PATCH][BUGFIX] cgroups: more safe tasklist locking in cgroup_attach_proc
    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>
    ---
    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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