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    SubjectRe: POLLRDONCE optimisation for epoll users (was: epoll and half closed TCP connections)
    Davide Libenzi wrote:
    > > > > (a) fd isn't a socket
    > > > > (b) fd isn't a TCP socket
    > > > > (c) kernel version <= 2.5.75
    > > > > (d) SO_RCVLOWAT < s
    > > > > (e) there is urgent data with OOBINLINE (I think)
    > >
    > > > Jamie, did you smoke that stuff again ? :)
    > > > With Eric patch in the proper places it is just fine. You just make
    > > > f_op->poll() to report the extra flag other that POLLIN. What's the problem ?
    > >
    > > The problem in cases (a)-(e) is your loop will call read() just once
    > > when it needs to call read() until it sees EAGAIN.
    > >
    > > What's wrong is the behaviour of your program when the extra flag
    > > _isn't_ set.
    >
    > Jamie, the loop will call read(2) until data is available. With the trick
    > of checking the returned number of bytes you can avoid the extra EAGAIN
    > read(2). That's the point of the read(2) trick.

    That _only_ works if none of those conditions (a)-(e) applies.
    Otherwise, short reads are possible when there is more to come.

    Sure, if you're willing to assert that the program is running on
    kernel >= 2.5.76, all its fds are for sure TCP sockets and you added
    the POLLPRI check, then yes it's fine.

    I think mine is better because it works always, and you are free to
    code the optimisation in any programs, libraries etc.

    > The final check for RDHUP will tell that it has no more to wait for
    > POLLINs since there's no more someone sending.

    Sure, _that_ check is fine.

    -- Jamie
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