On Wed, 2007-08-22 at 16:17 -0700, Paul Menage wrote:
> I think this is the right way to handle the lockdep false-positive in
> the current containers patches, but I'm not that familiar with lockdep
> so any suggestions for a better approach are welcomed.
>
>
> In order to avoid a false-positive lockdep warning, we lock the root
> inode of a new filesystem mount prior to taking container_mutex, to
> preserve the invariant that container_mutex nests inside
> inode->i_mutex. In order to prevent a lockdep false positive when
> locking i_mutex on a newly-created container directory inode we use
> mutex_lock_nested(), with a nesting level of I_MUTEX_CHILD since the
> new inode will ultimately be a child directory of the parent whose
> i_mutex is nested outside of container_mutex.
So the normal order is:
inode->i_mutex
container_mutex (I_MUTEX_NORMAL)
and there is a one off reversal where we take container_mutex before
inode->i_mutex, which is safe because the inode is fresh, so nobody
could have possibly taken it to form the deadlock.
mutex_lock_nested() is indeed the proper annotation.
Using I_MUTEX_CHILD forms the chain:
container_mutex
inode->i_mutex (I_MUTEX_CHILD)
In order for this to become a problem there would need to be a:
inode->i_mutex (I_MUTEX_CHILD)
inode->i_mutex (I_MUTEX_NORMAL)
chain, which I suppose is precluded to exists on basis of common
sense :-)
> Signed-off-by: Paul Menage <menage at google.com>
Acked-by: Peter Zijlstra <a.p.zijlstra at chello.nl>
>
> ---
> kernel/container.c | 17 +++++++----------
> 1 file changed, 7 insertions(+), 10 deletions(-)
>
> Index: container-2.6.23-rc3-mm1/kernel/container.c
> ===================================================================
> --- container-2.6.23-rc3-mm1.orig/kernel/container.c
> +++ container-2.6.23-rc3-mm1/kernel/container.c
> @@ -966,13 +966,16 @@ static int container_get_sb(struct file_
> } else {
> /* New superblock */
> struct container *cont = &root->top_container;
> + struct inode *inode;
>
> BUG_ON(sb->s_root != NULL);
>
> ret = container_get_rootdir(sb);
> if (ret)
> goto drop_new_super;
> + inode = sb->s_root->d_inode;
>
> + mutex_lock(&inode->i_mutex);
> mutex_lock(&container_mutex);
>
> /*
> @@ -985,12 +988,14 @@ static int container_get_sb(struct file_
> ret = allocate_cg_links(css_group_count, &tmp_cg_links);
> if (ret) {
> mutex_unlock(&container_mutex);
> + mutex_unlock(&inode->i_mutex);
> goto drop_new_super;
> }
>
> ret = rebind_subsystems(root, root->subsys_bits);
> if (ret == -EBUSY) {
> mutex_unlock(&container_mutex);
> + mutex_unlock(&inode->i_mutex);
> goto drop_new_super;
> }
>
> @@ -1030,16 +1035,8 @@ static int container_get_sb(struct file_
> BUG_ON(!list_empty(&cont->children));
> BUG_ON(root->number_of_containers != 1);
>
> - /*
> - * I believe that it's safe to nest i_mutex inside
> - * container_mutex in this case, since no-one else can
> - * be accessing this directory yet. But we still need
> - * to teach lockdep that this is the case - currently
> - * a containerfs remount triggers a lockdep warning
> - */
> - mutex_lock(&cont->dentry->d_inode->i_mutex);
> container_populate_dir(cont);
> - mutex_unlock(&cont->dentry->d_inode->i_mutex);
> + mutex_unlock(&inode->i_mutex);
> mutex_unlock(&container_mutex);
> }
>
> @@ -1529,7 +1526,7 @@ static int container_create_file(struct
>
> /* start with the directory inode held, so that we can
> * populate it without racing with another mkdir */
> - mutex_lock(&inode->i_mutex);
> + mutex_lock_nested(&inode->i_mutex, I_MUTEX_CHILD);
> } else if (S_ISREG(mode)) {
> inode->i_size = 0;
> inode->i_fop = &container_file_operations;
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