[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>

1 files changed, 51 insertions, 32 deletions

diff --git a/fs/select.c b/fs/select.c
index a8109baa5e46..9c4f0f2604f1 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;