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    Subject[PATCH 01/10] sysfs: Support for preventing unmounts.
    sysfs: Support for preventing unmounts.

    To support mounting multiple instances of sysfs occassionally I
    need to walk through all of the currently present sysfs super blocks.

    To allow this iteration this patch adds sysfs_grab_supers
    and sysfs_release_supers. While a piece of code is in
    a section surrounded by these no more sysfs super blocks
    will be either created or destroyed.

    Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
    Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
    ---
    fs/sysfs/mount.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++++------
    fs/sysfs/sysfs.h | 10 ++++++
    2 files changed, 81 insertions(+), 8 deletions(-)

    Index: linux-mm/fs/sysfs/mount.c
    ===================================================================
    --- linux-mm.orig/fs/sysfs/mount.c
    +++ linux-mm/fs/sysfs/mount.c
    @@ -41,47 +41,110 @@ struct sysfs_dirent sysfs_root = {

    static int sysfs_fill_super(struct super_block *sb, void *data, int silent)
    {
    - struct inode *inode;
    - struct dentry *root;
    + struct sysfs_super_info *info = NULL;
    + struct inode *inode = NULL;
    + struct dentry *root = NULL;
    + int error;

    sb->s_blocksize = PAGE_CACHE_SIZE;
    sb->s_blocksize_bits = PAGE_CACHE_SHIFT;
    sb->s_magic = SYSFS_MAGIC;
    sb->s_op = &sysfs_ops;
    sb->s_time_gran = 1;
    - sysfs_sb = sb;
    + if (!sysfs_sb)
    + sysfs_sb = sb;
    +
    + error = -ENOMEM;
    + info = kzalloc(sizeof(*info), GFP_KERNEL);
    + if (!info)
    + goto out_err;

    /* get root inode, initialize and unlock it */
    + error = -ENOMEM;
    inode = sysfs_get_inode(&sysfs_root);
    if (!inode) {
    pr_debug("sysfs: could not get root inode\n");
    - return -ENOMEM;
    + goto out_err;
    }

    /* instantiate and link root dentry */
    + error = -ENOMEM;
    root = d_alloc_root(inode);
    if (!root) {
    pr_debug("%s: could not get root dentry!\n",__func__);
    - iput(inode);
    - return -ENOMEM;
    + goto out_err;
    }
    root->d_fsdata = &sysfs_root;
    sb->s_root = root;
    + sb->s_fs_info = info;
    return 0;
    +
    +out_err:
    + dput(root);
    + iput(inode);
    + kfree(info);
    + if (sysfs_sb == sb)
    + sysfs_sb = NULL;
    + return error;
    }

    static int sysfs_get_sb(struct file_system_type *fs_type,
    int flags, const char *dev_name, void *data, struct vfsmount *mnt)
    {
    - return get_sb_single(fs_type, flags, data, sysfs_fill_super, mnt);
    + int rc;
    + mutex_lock(&sysfs_rename_mutex);
    + rc = get_sb_single(fs_type, flags, data, sysfs_fill_super, mnt);
    + mutex_unlock(&sysfs_rename_mutex);
    + return rc;
    }

    -static struct file_system_type sysfs_fs_type = {
    +struct file_system_type sysfs_fs_type = {
    .name = "sysfs",
    .get_sb = sysfs_get_sb,
    .kill_sb = kill_anon_super,
    };

    +void sysfs_grab_supers(void)
    +{
    + /* must hold sysfs_rename_mutex */
    + struct super_block *sb;
    + /* Loop until I have taken s_umount on all sysfs superblocks */
    +restart:
    + spin_lock(&sb_lock);
    + list_for_each_entry(sb, &sysfs_fs_type.fs_supers, s_instances) {
    + if (sysfs_info(sb)->grabbed)
    + continue;
    + /* Wait for unmount activity to complete. */
    + if (sb->s_count < S_BIAS) {
    + sb->s_count += 1;
    + spin_unlock(&sb_lock);
    + down_read(&sb->s_umount);
    + drop_super(sb);
    + goto restart;
    + }
    + atomic_inc(&sb->s_active);
    + sysfs_info(sb)->grabbed = 1;
    + }
    + spin_unlock(&sb_lock);
    +}
    +
    +void sysfs_release_supers(void)
    +{
    + /* must hold sysfs_rename_mutex */
    + struct super_block *sb;
    +restart:
    + spin_lock(&sb_lock);
    + list_for_each_entry(sb, &sysfs_fs_type.fs_supers, s_instances) {
    + if (!sysfs_info(sb)->grabbed)
    + continue;
    + sysfs_info(sb)->grabbed = 0;
    + spin_unlock(&sb_lock);
    + deactivate_super(sb);
    + goto restart;
    + }
    + spin_unlock(&sb_lock);
    +}
    +
    int __init sysfs_init(void)
    {
    int err = -ENOMEM;
    Index: linux-mm/fs/sysfs/sysfs.h
    ===================================================================
    --- linux-mm.orig/fs/sysfs/sysfs.h
    +++ linux-mm/fs/sysfs/sysfs.h
    @@ -85,6 +85,12 @@ struct sysfs_addrm_cxt {
    int cnt;
    };

    +struct sysfs_super_info {
    + int grabbed;
    +};
    +
    +#define sysfs_info(SB) ((struct sysfs_super_info *)(SB)->s_fs_info)
    +
    /*
    * mount.c
    */
    @@ -92,6 +98,10 @@ extern struct sysfs_dirent sysfs_root;
    extern struct super_block *sysfs_sb;
    extern struct kmem_cache *sysfs_dir_cachep;
    extern struct vfsmount *sysfs_mount;
    +extern struct file_system_type sysfs_fs_type;
    +
    +void sysfs_grab_supers(void);
    +void sysfs_release_supers(void);

    /*
    * dir.c
    --


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