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>

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index b5e43e11f1e37f0df761d818ed315a629444c828..88473e6271ef6840a964982b6466a77d8ccef7d1 100644 (file)
--- a/fs/eventpoll.c
+++ b/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