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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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