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    Subject[PATCH] sched: fix another race when reading /proc/sched_debug
    I fixed an oops with the following commit:

    | commit 24eb089950ce44603b30a3145a2c8520e2b55bb1
    | Author: Li Zefan <lizf@cn.fujitsu.com>
    | Date: Thu Nov 6 12:53:32 2008 -0800
    |
    | cgroups: fix invalid cgrp->dentry before cgroup has been completely removed
    |
    | This fixes an oops when reading /proc/sched_debug.

    The above commit fixed a race that reading /proc/sched_debug may access
    NULL cgrp->dentry if a cgroup is being removed.

    But I found there's another different race, in that reading sched_debug
    may access a cgroup which hasn't been completed created, and thus
    dereference NULL cgrp->dentry!

    cgroup_create()
    cpu_cgroup_create()
    register_fair_sched_group()
    list_add_rcu(...)
    print_cfs_stats()
    for_each_leaf_cfs_rq()
    print_cfs_rq()
    cgroup_path()
    cgroup->dentry = dentry;

    task_group is added to the global list before the cgroup has been created
    completely, if at this time print_cfs_stats() is called, it will access
    the half-created cgroup.

    This patch fixes the bug by holding cgroup_lock() to wait the cgroup to
    be created completely before calling cgroup_path().

    Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
    ---

    The patch is based on linus's git tree, and should go into 2.6.28,
    but it conflicts with the cleanup patch in sched/core:
    0a0db8f5c9d4bbb9bbfcc2b6cb6bce2d0ef4d73d

    ---
    kernel/sched_debug.c | 10 ++++++++--
    1 files changed, 8 insertions(+), 2 deletions(-)

    diff --git a/kernel/sched_debug.c b/kernel/sched_debug.c
    index 26ed8e3..01abf5b 100644
    --- a/kernel/sched_debug.c
    +++ b/kernel/sched_debug.c
    @@ -127,8 +127,11 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
    if (tg)
    cgroup = tg->css.cgroup;

    - if (cgroup)
    + if (cgroup) {
    + cgroup_lock();
    cgroup_path(cgroup, path, sizeof(path));
    + cgroup_unlock();
    + }

    SEQ_printf(m, "\ncfs_rq[%d]:%s\n", cpu, path);
    #else
    @@ -181,8 +184,11 @@ void print_rt_rq(struct seq_file *m, int cpu, struct rt_rq *rt_rq)
    if (tg)
    cgroup = tg->css.cgroup;

    - if (cgroup)
    + if (cgroup) {
    + cgroup_lock();
    cgroup_path(cgroup, path, sizeof(path));
    + cgroup_unlock();
    + }

    SEQ_printf(m, "\nrt_rq[%d]:%s\n", cpu, path);
    #else
    --
    1.5.4.rc3

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