fs/eventpoll.c: loosen irq safety in ep_poll()

Similar to other calls, ep_poll() is not called with interrupts disabled,
and we can therefore avoid the irq save/restore dance and just disable
local irqs.  In fact, the call should never be called in irq context at
all, considering that the only path is

epoll_wait(2) -> do_epoll_wait() -> ep_poll().

When running on a 2 socket 40-core (ht) IvyBridge a common pipe based
epoll_wait(2) microbenchmark, the following performance improvements are
seen:

    # threads       vanilla         dirty
	 1          1805587	    2106412
	 2          1854064	    2090762
	 4          1805484	    2017436
	 8          1751222	    1974475
	 16         1725299	    1962104
	 32         1378463	    1571233
	 64          787368	     900784

Which is a pretty constantly near 15%.

Also add a lockdep check such that we detect any mischief before
deadlocking.

Link: http://lkml.kernel.org/r/20180727053432.16679-2-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jason Baron <jbaron@akamai.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/eventpoll.c

@@ -1746,11 +1746,12 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		   int maxevents, long timeout)
 {
 	int res = 0, eavail, timed_out = 0;
-	unsigned long flags;
 	u64 slack = 0;
 	wait_queue_entry_t wait;
 	ktime_t expires, *to = NULL;
 
+	lockdep_assert_irqs_enabled();
+
 	if (timeout > 0) {
 		struct timespec64 end_time = ep_set_mstimeout(timeout);
 
@@ -1763,7 +1764,7 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		 * caller specified a non blocking operation.
 		 */
 		timed_out = 1;
-		spin_lock_irqsave(&ep->wq.lock, flags);
+		spin_lock_irq(&ep->wq.lock);
 		goto check_events;
 	}
 
@@ -1772,7 +1773,7 @@ fetch_events:
 	if (!ep_events_available(ep))
 		ep_busy_loop(ep, timed_out);
 
-	spin_lock_irqsave(&ep->wq.lock, flags);
+	spin_lock_irq(&ep->wq.lock);
 
 	if (!ep_events_available(ep)) {
 		/*
@@ -1814,11 +1815,11 @@ fetch_events:
 				break;
 			}
 
-			spin_unlock_irqrestore(&ep->wq.lock, flags);
+			spin_unlock_irq(&ep->wq.lock);
 			if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS))
 				timed_out = 1;
 
-			spin_lock_irqsave(&ep->wq.lock, flags);
+			spin_lock_irq(&ep->wq.lock);
 		}
 
 		__remove_wait_queue(&ep->wq, &wait);
@@ -1828,7 +1829,7 @@ check_events:
 	/* Is it worth to try to dig for events ? */
 	eavail = ep_events_available(ep);
 
-	spin_unlock_irqrestore(&ep->wq.lock, flags);
+	spin_unlock_irq(&ep->wq.lock);
 
 	/*
 	 * Try to transfer events to user space. In case we get 0 events and