[PATCH 2/2] fs/epoll: deal with wait_queue only once

[Davidlohr Bueso]

There is no reason why we rearm the waitiqueue upon every
fetch_events retry (for when events are found yet send_events()
fails). If nothing else, this saves four lock operations per
retry, and furthermore reduces the scope of the lock even
further.

Signed-off-by: Davidlohr Bueso <dbu...@suse.de>
---
 fs/eventpoll.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 7e2b5f3d6b3e..25b0c94cc091 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1749,6 +1749,7 @@ static int ep_poll(struct eventpoll *ep, struct 
epoll_event __user *events,
 {
        int res = 0, eavail, timed_out = 0;
        u64 slack = 0;
+       bool waiter = false;
        wait_queue_entry_t wait;
        ktime_t expires, *to = NULL;
 
@@ -1786,6 +1787,15 @@ static int ep_poll(struct eventpoll *ep, struct 
epoll_event __user *events,
        if (eavail)
                goto send_events;
 
+       if (!waiter) {
+               waiter = true;
+               init_waitqueue_entry(&wait, current);
+
+               spin_lock_irq(&ep->wq.lock);
+               __add_wait_queue_exclusive(&ep->wq, &wait);
+               spin_unlock_irq(&ep->wq.lock);
+       }
+
        /*
         * Busy poll timed out.  Drop NAPI ID for now, we can add
         * it back in when we have moved a socket with a valid NAPI
@@ -1798,10 +1808,6 @@ static int ep_poll(struct eventpoll *ep, struct 
epoll_event __user *events,
         * We need to sleep here, and we will be wake up by
         * ep_poll_callback() when events will become available.
         */
-       init_waitqueue_entry(&wait, current);
-       spin_lock_irq(&ep->wq.lock);
-       __add_wait_queue_exclusive(&ep->wq, &wait);
-       spin_unlock_irq(&ep->wq.lock);
 
        for (;;) {
                /*
@@ -1836,10 +1842,6 @@ static int ep_poll(struct eventpoll *ep, struct 
epoll_event __user *events,
 
        __set_current_state(TASK_RUNNING);
 
-       spin_lock_irq(&ep->wq.lock);
-       __remove_wait_queue(&ep->wq, &wait);
-       spin_unlock_irq(&ep->wq.lock);
-
 send_events:
        /*
         * Try to transfer events to user space. In case we get 0 events and
@@ -1850,6 +1852,12 @@ static int ep_poll(struct eventpoll *ep, struct 
epoll_event __user *events,
            !(res = ep_send_events(ep, events, maxevents)) && !timed_out)
                goto fetch_events;
 
+       if (waiter) {
+               spin_lock_irq(&ep->wq.lock);
+               __remove_wait_queue(&ep->wq, &wait);
+               spin_unlock_irq(&ep->wq.lock);
+       }
+
        return res;
 }
 
-- 
2.16.4