• Jeff Layton's avatar
    locks: add new fcntl cmd values for handling file private locks · 5d50ffd7
    Jeff Layton authored
    Due to some unfortunate history, POSIX locks have very strange and
    unhelpful semantics. The thing that usually catches people by surprise
    is that they are dropped whenever the process closes any file descriptor
    associated with the inode.
    
    This is extremely problematic for people developing file servers that
    need to implement byte-range locks. Developers often need a "lock
    management" facility to ensure that file descriptors are not closed
    until all of the locks associated with the inode are finished.
    
    Additionally, "classic" POSIX locks are owned by the process. Locks
    taken between threads within the same process won't conflict with one
    another, which renders them useless for synchronization between threads.
    
    This patchset adds a new type of lock that attempts to address these
    issues. These locks conflict with classic POSIX read/write locks, but
    have semantics that are more like BSD locks with respect to inheritance
    and behavior on close.
    
    This is implemented primarily by changing how fl_owner field is set for
    these locks. Instead of having them owned by the files_struct of the
    process, they are instead owned by the filp on which they were acquired.
    Thus, they are inherited across fork() and are only released when the
    last reference to a filp is put.
    
    These new semantics prevent them from being merged with classic POSIX
    locks, even if they are acquired by the same process. These locks will
    also conflict with classic POSIX locks even if they are acquired by
    the same process or on the same file descriptor.
    
    The new locks are managed using a new set of cmd values to the fcntl()
    syscall. The initial implementation of this converts these values to
    "classic" cmd values at a fairly high level, and the details are not
    exposed to the underlying filesystem. We may eventually want to push
    this handing out to the lower filesystem code but for now I don't
    see any need for it.
    
    Also, note that with this implementation the new cmd values are only
    available via fcntl64() on 32-bit arches. There's little need to
    add support for legacy apps on a new interface like this.
    Signed-off-by: 's avatarJeff Layton <jlayton@redhat.com>
    5d50ffd7
fcntl.c 16.6 KB