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    cgroups: fix 2.6.32 regression causing BUG_ON() in cgroup_diput() · bd4f490a
    Dave Anderson 提交于
    The LTP cgroup test suite generates a "kernel BUG at kernel/cgroup.c:790!"
    here in cgroup_diput():
    
                     /*
                      * if we're getting rid of the cgroup, refcount should ensure
                      * that there are no pidlists left.
                      */
                     BUG_ON(!list_empty(&cgrp->pidlists));
    
    The cgroup pidlist rework in 2.6.32 generates the BUG_ON, which is caused
    when pidlist_array_load() calls cgroup_pidlist_find():
    
    (1) if a matching cgroup_pidlist is found, it down_write's the mutex of the
         pre-existing cgroup_pidlist, and increments its use_count.
    (2) if no matching cgroup_pidlist is found, then a new one is allocated, it
         down_write's its mutex, and the use_count is set to 0.
    (3) the matching, or new, cgroup_pidlist gets returned back to pidlist_array_load(),
         which increments its use_count -- regardless whether new or pre-existing --
         and up_write's the mutex.
    
    So if a matching list is ever encountered by cgroup_pidlist_find() during
    the life of a cgroup directory, it results in an inflated use_count value,
    preventing it from ever getting released by cgroup_release_pid_array().
    Then if the directory is subsequently removed, cgroup_diput() hits the
    BUG_ON() when it finds that the directory's cgroup is still populated with
    a pidlist.
    
    The patch simply removes the use_count increment when a matching pidlist
    is found by cgroup_pidlist_find(), because it gets bumped by the calling
    pidlist_array_load() function while still protected by the list's mutex.
    Signed-off-by: NDave Anderson <anderson@redhat.com>
    Reviewed-by: NLi Zefan <lizf@cn.fujitsu.com>
    Acked-by: NBen Blum <bblum@andrew.cmu.edu>
    Cc: Paul Menage <menage@google.com>
    Cc: <stable@kernel.org>
    Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
    bd4f490a
cgroup.c 108.5 KB