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    SubjectRe: [PATCH] sched: fix another race when reading /proc/sched_debug

    * Li Zefan <lizf@cn.fujitsu.com> wrote:

    > 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>

    applied to tip/sched/urgent, thanks!

    > 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

    i merged it up in tip/master, could you please check whether it's ok?

    Ingo


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