From 4a4b69f79ba7286794765a856349e380f984a6cb Mon Sep 17 00:00:00 2001
From: Vadim Lobanov <vlobanov@speakeasy.net>
Date: Fri, 23 Jun 2006 02:05:16 -0700
Subject: [PATCH] [PATCH] Poll cleanups/microoptimizations

The "count" and "pt" variables are declared and modified by do_poll(), as
well as accessed and written indirectly in the do_pollfd() subroutine.

This patch pulls all handling of these variables into the do_poll()
function, thereby eliminating the odd use of indirection in do_pollfd().
This is done by pulling the "struct pollfd" traversal loop from do_pollfd()
into its only caller do_poll().  As an added bonus, the patch saves a few
clock cycles, and also adds comments to make the code easier to follow.

Signed-off-by: Vadim Lobanov <vlobanov@speakeasy.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
---
 fs/select.c | 83 ++++++++++++++++++++++++++++++++---------------------
 1 file changed, 51 insertions(+), 32 deletions(-)

diff --git a/fs/select.c b/fs/select.c
index a8109baa5e..9c4f0f2604 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -546,37 +546,38 @@ struct poll_list {
 
 #define POLLFD_PER_PAGE  ((PAGE_SIZE-sizeof(struct poll_list)) / sizeof(struct pollfd))
 
-static void do_pollfd(unsigned int num, struct pollfd * fdpage,
-	poll_table ** pwait, int *count)
+/*
+ * Fish for pollable events on the pollfd->fd file descriptor. We're only
+ * interested in events matching the pollfd->events mask, and the result
+ * matching that mask is both recorded in pollfd->revents and returned. The
+ * pwait poll_table will be used by the fd-provided poll handler for waiting,
+ * if non-NULL.
+ */
+static inline unsigned int do_pollfd(struct pollfd *pollfd, poll_table *pwait)
 {
-	int i;
-
-	for (i = 0; i < num; i++) {
-		int fd;
-		unsigned int mask;
-		struct pollfd *fdp;
-
-		mask = 0;
-		fdp = fdpage+i;
-		fd = fdp->fd;
-		if (fd >= 0) {
-			int fput_needed;
-			struct file * file = fget_light(fd, &fput_needed);
-			mask = POLLNVAL;
-			if (file != NULL) {
-				mask = DEFAULT_POLLMASK;
-				if (file->f_op && file->f_op->poll)
-					mask = file->f_op->poll(file, *pwait);
-				mask &= fdp->events | POLLERR | POLLHUP;
-				fput_light(file, fput_needed);
-			}
-			if (mask) {
-				*pwait = NULL;
-				(*count)++;
-			}
+	unsigned int mask;
+	int fd;
+
+	mask = 0;
+	fd = pollfd->fd;
+	if (fd >= 0) {
+		int fput_needed;
+		struct file * file;
+
+		file = fget_light(fd, &fput_needed);
+		mask = POLLNVAL;
+		if (file != NULL) {
+			mask = DEFAULT_POLLMASK;
+			if (file->f_op && file->f_op->poll)
+				mask = file->f_op->poll(file, pwait);
+			/* Mask out unneeded events. */
+			mask &= pollfd->events | POLLERR | POLLHUP;
+			fput_light(file, fput_needed);
 		}
-		fdp->revents = mask;
 	}
+	pollfd->revents = mask;
+
+	return mask;
 }
 
 static int do_poll(unsigned int nfds,  struct poll_list *list,
@@ -594,11 +595,29 @@ static int do_poll(unsigned int nfds,  struct poll_list *list,
 		long __timeout;
 
 		set_current_state(TASK_INTERRUPTIBLE);
-		walk = list;
-		while(walk != NULL) {
-			do_pollfd( walk->len, walk->entries, &pt, &count);
-			walk = walk->next;
+		for (walk = list; walk != NULL; walk = walk->next) {
+			struct pollfd * pfd, * pfd_end;
+
+			pfd = walk->entries;
+			pfd_end = pfd + walk->len;
+			for (; pfd != pfd_end; pfd++) {
+				/*
+				 * Fish for events. If we found one, record it
+				 * and kill the poll_table, so we don't
+				 * needlessly register any other waiters after
+				 * this. They'll get immediately deregistered
+				 * when we break out and return.
+				 */
+				if (do_pollfd(pfd, pt)) {
+					count++;
+					pt = NULL;
+				}
+			}
 		}
+		/*
+		 * All waiters have already been registered, so don't provide
+		 * a poll_table to them on the next loop iteration.
+		 */
 		pt = NULL;
 		if (count || !*timeout || signal_pending(current))
 			break;
-- 
2.39.5