• Serge E. Hallyn's avatar
    namespaces: ipc namespaces: implement support for posix msqueues · 7eafd7c7
    Serge E. Hallyn authored
    Implement multiple mounts of the mqueue file system, and link it to usage
    Each ipc ns has a corresponding mqueuefs superblock.  When a user does
    clone(CLONE_NEWIPC) or unshare(CLONE_NEWIPC), the unshare will cause an
    internal mount of a new mqueuefs sb linked to the new ipc ns.
    When a user does 'mount -t mqueue mqueue /dev/mqueue', he mounts the
    mqueuefs superblock.
    Posix message queues can be worked with both through the mq_* system calls
    (see mq_overview(7)), and through the VFS through the mqueue mount.  Any
    usage of mq_open() and friends will work with the acting task's ipc
    namespace.  Any actions through the VFS will work with the mqueuefs in
    which the file was created.  So if a user doesn't remount mqueuefs after
    unshare(CLONE_NEWIPC), mq_open("/ab") will not be reflected in "ls
    If task a mounts mqueue for ipc_ns:1, then clones task b with a new ipcns,
    ipcns:2, and then task a is the last task in ipc_ns:1 to exit, then (1)
    ipc_ns:1 will be freed, (2) it's superblock will live on until task b
    umounts the corresponding mqueuefs, and vfs actions will continue to
    succeed, but (3) sb->s_fs_info will be NULL for the sb corresponding to
    the deceased ipc_ns:1.
    To make this happen, we must protect the ipc reference count when
    a) a task exits and drops its ipcns->count, since it might be dropping
       it to 0 and freeing the ipcns
    b) a task accesses the ipcns through its mqueuefs interface, since it
       bumps the ipcns refcount and might race with the last task in the ipcns
    So the kref is changed to an atomic_t so we can use
    atomic_dec_and_lock(&ns->count,mq_lock), and every access to the ipcns
    through ns = mqueuefs_sb->s_fs_info is protected by the same lock.
    Signed-off-by: default avatarCedric Le Goater <clg@fr.ibm.com>
    Signed-off-by: default avatarSerge E. Hallyn <serue@us.ibm.com>
    Cc: Alexey Dobriyan <adobriyan@gmail.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
util.h 5.12 KB