On Mon, Dec 30, 2002 at 12:21:28PM +0800, zw wrote: > I seem to understand, but I don't. > > In the case of tcp_poll(). From sys_poll() all the way down to just > before the call to tcp_poll() through a file pointer, kernel setup > the poll_wqueue all well. And then just as soon as the entering of > tcp_poll(), it calls poll_wait() to wakeup the queue. What's the > point? It hasn't done anything real just yet. > > Ie., first setup a wait_queue, then wakeup the queue, then to do the > real work. What's the point of the wait_queue? IIRC poll_wait is not a wake-up at all. It ends in add_wait_queue (I am looking at 2.4.19, but I don't think poll logic changed that much). IIRC the poll logic goes as follows: sys_poll calls ->poll method on all descriptors. These call poll_wait to set up wait queues and then check the current state. If all ->poll methods return 0, sys_poll schedule()s and goes to call the methods again. If some return nonzero or time is up, it discards the queues and returns. ------------------------------------------------------------------------------- Jan 'Bulb' Hudec <bulb@ucw.cz> -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
