lkml.org 
[lkml]   [2003]   [Jul]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [Patch][RFC] epoll and half closed TCP connections
    > > I guess my only argument would be that edge triggered mode isn't really
    > > workable with TCP connections if there's no way to solve the ambiguity
    > > between EOF and no data in buffer (at least w/o an extra syscall). I just
    > > realized that the race you mention in the man page (reading data from
    > > the 'next' event that hasn't been polled into user mode yet) will lead to
    > > the same issue: how do you know if you got this event because you consumed
    > > the data on the previous interrupt or if this is an EOF condition.
    >
    > (Sorry, I missed this)
    > You can work that out very easily. When your read/write returns a lower
    > number of bytes, it means that it is time to stop processing this fd. If
    > events happened meanwhile, you will get them at the next epoll_wait(). If
    > not, the next time they'll happen. There's no blind spot if you follow
    > this simple rule, and you do not even have the extra syscall with EAGAIN.

    The scenario that I think is still uncovered (edge trigger only):

    User Kernel
    -------- ----------
    Read data added to socket

    Socket posts read event to epfd

    epoll_wait() Event cleared from epfd, EPOLLIN
    returned to user

    more read data added to socket

    Socket posts a new read event to epfd

    read() until short read with EAGAIN all data read from socket

    epoll_wait() returns another EPOLLIN for socket and
    clears it from epfd

    read(), returns 0 right away socket buffer is empty

    This is your 'false positive' case in the epoll(4) man page.

    How does the app tell the 0 read here from a read EOF coming from the remote?

    If it assumes this is a false positive and there was also an EOF
    indication, the EOF will be lost; if it assumes it an EOF the connection
    will be prematurely terminated.

    -Eric
    -
    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: 2005-03-22 13:46    [W:2.956 / U:0.028 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site