lkml.org 
[lkml]   [2007]   [Oct]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    SubjectRE: epoll design problems with common fork/exec patterns
    Date

    > 6) Epoll removes the file from the set, when the *kernel* object gets
    > closed (internal use-count goes to zero)
    >
    > With that in mind, how can the code snippet above trigger a removal from
    > the epoll set?

    I don't see how that can be. Suppose I add fd 8 to an epoll set. Suppose fd
    5 is a dup of fd 8. Now, I close fd 8. How can fd 8 remain in my epoll set,
    since there no longer is an fd 8? Events on files registered for epoll
    notification are reported by descriptor, so the set membership has to be
    associated (as reflected into userspace) with the descriptor, not the file.

    For example, consider:

    1) Process creates an epoll set, the set gets fd 4.

    2) Process creates a socket, it gets fd 5.

    3) The process adds fd 5 to set 4.

    4) The process forks.

    5) The child inherits the epoll set but not the socket.

    Here the kernel cannot quite do the right thing. Ideally, the parent would
    still have fd 5 in its version of the epoll set. After all, it has not
    closed fd 5. However, the child *cannot* see fd 5 in its version of the
    epoll set since it has no fd 5. An event reported for fd 5 would be
    nonsense.

    So it seems the kernel either has to break one of these "would/cannot"
    requirements, or it has to split the epoll set in two. However, splitting
    the set into two sets is clearly wrong since the processes should share it.

    Q6 Will the close of an fd cause it to be removed from
    all
    epoll sets automatically?

    A6 Yes.

    Note that this talks of the close of an "fd", not a file. The 'close'
    function in fact closes an fd, as that fd is then reusable. So it sounds
    like the problem above is solved by removing the fd from the set, but in
    practice this doesn't happen. I have programs that call 'close' between
    'fork' and 'exec' and do not see the socket removed from the poll set.

    DS


    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2007-10-28 05:49    [W:0.025 / U:59.208 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site