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    SubjectRe: [PATCH] cgroup: fix reverse unlock sequence in cgroup_get_sb
    * Zefan Li <lizf.kernel@gmail.com> [2009-07-21 19:38:03]:

    > 2009/7/21, Balbir Singh <balbir@linux.vnet.ibm.com>:
    > >
    > > * Xiaotian Feng <dfeng@redhat.com> [2009-07-21 18:25:26]:
    > >
    > > > In cgroup_get_sb, the lock sequence is:
    > > > mutex_lock(&inode->i_mutex);
    > > > mutex_lock(&cgroup->mutex);
    > > > so the last unlock sequence should be:
    > > > mutex_unlock(&cgroup->mutex);
    > > > mutex_unlock(&inode->i_mutex);
    > > >
    > > > Signed-off-by: Xiaotian Feng <dfeng@redhat.com>
    > > > ---
    > > > kernel/cgroup.c | 2 +-
    > > > 1 files changed, 1 insertions(+), 1 deletions(-)
    > > >
    > > > diff --git a/kernel/cgroup.c b/kernel/cgroup.c
    > > > index 3737a68..11ef162 100644
    > > > --- a/kernel/cgroup.c
    > > > +++ b/kernel/cgroup.c
    > > > @@ -1140,8 +1140,8 @@ static int cgroup_get_sb(struct file_system_type
    > > *fs_type,
    > > > BUG_ON(root->number_of_cgroups != 1);
    > > >
    > > > cgroup_populate_dir(root_cgrp);
    > > > - mutex_unlock(&inode->i_mutex);
    > > > mutex_unlock(&cgroup_mutex);
    > > > + mutex_unlock(&inode->i_mutex);
    > > > }
    > > >
    > >
    > > Seems reasonable to me. You might also want to mention that elsewhere
    > > the sequence is unlock cgroup_mutex followed by inode->i_mutex.
    > >
    > > Acked-by: Balbir Singh balbir@linux.vnet.ibm.com
    >
    >
    > No, the unlock order is irrelevant. It's the lock order that matters. So
    > this patch
    > fixes nothing.
    >
    > Xiaotian, you didn't run into deadlock, did you?
    >


    Li, Consider the following


    lock(A)
    lock(B)
    unlock(A)
    unlock(B)

    Tomorrow if a unsuspecting programmer does this

    lock(A)
    lock(B)
    unlock(A)

    code block

    unlock(B)


    What protects code block? lock B? Is that the intention?

    --
    Balbir


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