epoll: fix compiler warning and optimize the non-blocking path

Add a comment to ep_poll(), rename labels a bit clearly, fix a warning of
unused variable from gcc and optimize the non-blocking path a little.

Hinted-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Davide Libenzi <davidel@xmailserver.org>

hannes@cmpxchg.org:

: The non-blocking ep_poll path optimization introduced skipping over the
: return value setup.
:
: Initialize it properly, my userspace gets upset by epoll_wait() returning
: random things.
:
: In addition, remove the reinitialization at the fetch_events label, the
: return value is garuanteed to be zero when execution reaches there.

[hannes@cmpxchg.org: fix initialization]
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Shawn Bohrer <shawn.bohrer@gmail.com>
Acked-by: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 28 insertions, 5 deletions

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 57298d092f56..ed38801b57a7 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1148,12 +1148,29 @@ static inline struct timespec ep_set_mstimeout(long ms)
         return timespec_add_safe(now, ts);
 }
 
+/**
+ * ep_poll - Retrieves ready events, and delivers them to the caller supplied
+ *           event buffer.
+ *
+ * @ep: Pointer to the eventpoll context.
+ * @events: Pointer to the userspace buffer where the ready events should be
+ *          stored.
+ * @maxevents: Size (in terms of number of events) of the caller event buffer.
+ * @timeout: Maximum timeout for the ready events fetch operation, in
+ *           milliseconds. If the @timeout is zero, the function will not block,
+ *           while if the @timeout is less than zero, the function will block
+ *           until at least one event has been retrieved (or an error
+ *           occurred).
+ *
+ * Returns: Returns the number of ready events which have been fetched, or an
+ *          error code, in case of error.
+ */
 static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
                    int maxevents, long timeout)
 {
-        int res, eavail, timed_out = 0;
+        int res = 0, eavail, timed_out = 0;
         unsigned long flags;
-        long slack;
+        long slack = 0;
         wait_queue_t wait;
         ktime_t expires, *to = NULL;
 
@@ -1164,13 +1181,18 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
                 to = &expires;
                 *to = timespec_to_ktime(end_time);
         } else if (timeout == 0) {
+                /*
+                 * Avoid the unnecessary trip to the wait queue loop, if the
+                 * caller specified a non blocking operation.
+                 */
                 timed_out = 1;
+                spin_lock_irqsave(&ep->lock, flags);
+                goto check_events;
         }
 
-retry:
+fetch_events:
         spin_lock_irqsave(&ep->lock, flags);
 
-        res = 0;
         if (!ep_events_available(ep)) {
                 /*
                  * We don't have any available event to return to the caller.
@@ -1204,6 +1226,7 @@ retry:
 
                 set_current_state(TASK_RUNNING);
         }
+check_events:
         /* Is it worth to try to dig for events ? */
         eavail = ep_events_available(ep);
 
@@ -1216,7 +1239,7 @@ retry:
          */
         if (!res && eavail &&
             !(res = ep_send_events(ep, events, maxevents)) && !timed_out)
-                goto retry;
+                goto fetch_events;
 
         return res;
 }