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    SubjectRe: epoll vs stdin/stdout
    On Mon, 7 Jul 2003, Davide Libenzi wrote:

    > On Mon, 7 Jul 2003, Eric Varsanyi wrote:
    >
    > > On Mon, Jul 07, 2003 at 11:57:02AM -0700, Davide Libenzi wrote:
    > > > Events caught by epoll comes from a file* since that is the abstraction
    > > > the kernel handles. Events really happen on the file* and there's no way
    > > > if you dup()ing 1000 times a single fd, to say that events are for fd = 122.
    > >
    > > It is useful/mildly common at the app level to want to get read events
    > > for fd0 and write avail events for fd1. An app that might want to deal
    > > with reads from stdin in one process and writes to stdout in another
    > > (something like "team" perhaps) would have trouble here too.
    > >
    > > Epoll's API/impl is great as it is IMO, not suggesting need for change, I was
    > > hoping there was a good standard trick someone worked up to get around
    > > this specifc end case of stdin/stdout usually being dups but sometimes
    > > not. Porting my event system over to use epoll was easy/straightforward
    > > except for this one minor hitch.
    > >
    > > I considered:
    > > - using a second epoll object just for one of the fd's (to inspire
    > > delivery of the event to 2 wait queues in the kernel); a little
    > > ugly because of need to stack another epfd under the main one
    > > just for stdout write events
    > >
    > > - select() on (0, 1, epfd) and just use epoll with a timeout of 0
    > > when select fires to gather bulk of events; morally similar to
    > > previous but using select (which I want to just get away from)
    > >
    > > - make the app use stdin as its output (this is what I ended up doing);
    > > breaks redirection of stdout but that doesn't matter to this app
    >
    > Any of the above. Pls wait for an incoming patch ...

    Try out this one, either over 2.5.74 or over an existing epoll-patched
    2.4.{20,21} ...



    - Davide




    diff -Nru linux-2.5.74.vanilla/fs/eventpoll.c linux-2.5.74.epoll/fs/eventpoll.c
    --- linux-2.5.74.vanilla/fs/eventpoll.c 2003-07-07 15:22:37.000000000 -0700
    +++ linux-2.5.74.epoll/fs/eventpoll.c 2003-07-07 15:40:44.000000000 -0700
    @@ -245,6 +245,9 @@
    /* The "container" of this item */
    struct eventpoll *ep;

    + /* The file descriptor this item refers to */
    + int fd;
    +
    /* The file this item refers to */
    struct file *file;

    @@ -285,15 +288,17 @@
    static int ep_alloc_pages(char **pages, int numpages);
    static int ep_free_pages(char **pages, int numpages);
    static int ep_file_init(struct file *file, unsigned int hashbits);
    -static unsigned int ep_hash_index(struct eventpoll *ep, struct file *file);
    +static unsigned int ep_hash_index(struct eventpoll *ep, struct file *file, int fd);
    static struct list_head *ep_hash_entry(struct eventpoll *ep, unsigned int index);
    static int ep_init(struct eventpoll *ep, unsigned int hashbits);
    static void ep_free(struct eventpoll *ep);
    -static struct epitem *ep_find(struct eventpoll *ep, struct file *file);
    +static struct epitem *ep_find(struct eventpoll *ep, struct file *file, int fd);
    static void ep_use_epitem(struct epitem *epi);
    static void ep_release_epitem(struct epitem *epi);
    -static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead, poll_table *pt);
    -static int ep_insert(struct eventpoll *ep, struct epoll_event *event, struct file *tfile);
    +static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
    + poll_table *pt);
    +static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
    + struct file *tfile, int fd);
    static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_event *event);
    static void ep_unregister_pollwait(struct eventpoll *ep, struct epitem *epi);
    static int ep_unlink(struct eventpoll *ep, struct epitem *epi);
    @@ -580,7 +585,7 @@
    down_write(&ep->sem);

    /* Try to lookup the file inside our hash table */
    - epi = ep_find(ep, tfile);
    + epi = ep_find(ep, tfile, fd);

    error = -EINVAL;
    switch (op) {
    @@ -588,7 +593,7 @@
    if (!epi) {
    epds.events |= POLLERR | POLLHUP;

    - error = ep_insert(ep, &epds, tfile);
    + error = ep_insert(ep, &epds, tfile, fd);
    } else
    error = -EEXIST;
    break;
    @@ -814,10 +819,11 @@
    /*
    * Calculate the index of the hash relative to "file".
    */
    -static unsigned int ep_hash_index(struct eventpoll *ep, struct file *file)
    +static unsigned int ep_hash_index(struct eventpoll *ep, struct file *file, int fd)
    {
    + unsigned long ptr = (unsigned long) file ^ (fd << ep->hashbits);

    - return (unsigned int) hash_ptr(file, ep->hashbits);
    + return (unsigned int) hash_ptr((void *) ptr, ep->hashbits);
    }


    @@ -920,7 +926,7 @@
    * the returned item, so the caller must call ep_release_epitem()
    * after finished using the "struct epitem".
    */
    -static struct epitem *ep_find(struct eventpoll *ep, struct file *file)
    +static struct epitem *ep_find(struct eventpoll *ep, struct file *file, int fd)
    {
    unsigned long flags;
    struct list_head *lsthead, *lnk;
    @@ -928,11 +934,11 @@

    read_lock_irqsave(&ep->lock, flags);

    - lsthead = ep_hash_entry(ep, ep_hash_index(ep, file));
    + lsthead = ep_hash_entry(ep, ep_hash_index(ep, file, fd));
    list_for_each(lnk, lsthead) {
    epi = list_entry(lnk, struct epitem, llink);

    - if (epi->file == file) {
    + if (epi->file == file && epi->fd == fd) {
    ep_use_epitem(epi);
    break;
    }
    @@ -976,7 +982,8 @@
    * This is the callback that is used to add our wait queue to the
    * target file wakeup lists.
    */
    -static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead, poll_table *pt)
    +static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
    + poll_table *pt)
    {
    struct epitem *epi = EP_ITEM_FROM_EPQUEUE(pt);
    struct eppoll_entry *pwq;
    @@ -995,7 +1002,8 @@
    }


    -static int ep_insert(struct eventpoll *ep, struct epoll_event *event, struct file *tfile)
    +static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
    + struct file *tfile, int fd)
    {
    int error, revents, pwake = 0;
    unsigned long flags;
    @@ -1014,6 +1022,7 @@
    INIT_LIST_HEAD(&epi->pwqlist);
    epi->ep = ep;
    epi->file = tfile;
    + epi->fd = fd;
    epi->event = *event;
    atomic_set(&epi->usecnt, 1);
    epi->nwait = 0;
    @@ -1046,7 +1055,7 @@
    write_lock_irqsave(&ep->lock, flags);

    /* Add the current item to the hash table */
    - list_add(&epi->llink, ep_hash_entry(ep, ep_hash_index(ep, tfile)));
    + list_add(&epi->llink, ep_hash_entry(ep, ep_hash_index(ep, tfile, fd)));

    /* If the file is already "ready" we drop it inside the ready list */
    if ((revents & event->events) && !EP_IS_LINKED(&epi->rdllink)) {
    @@ -1065,8 +1074,8 @@
    if (pwake)
    ep_poll_safewake(&psw, &ep->poll_wait);

    - DNPRINTK(3, (KERN_INFO "[%p] eventpoll: ep_insert(%p, %p)\n",
    - current, ep, tfile));
    + DNPRINTK(3, (KERN_INFO "[%p] eventpoll: ep_insert(%p, %p, %d)\n",
    + current, ep, tfile, fd));

    return 0;

    -
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