epoll: simplify ep_poll_safewake() for CONFIG_DEBUG_LOCK_ALLOC

Currently, ep_poll_safewake() in the CONFIG_DEBUG_LOCK_ALLOC case uses
ep_call_nested() in order to pass the correct subclass argument to
spin_lock_irqsave_nested().  However, ep_call_nested() adds unnecessary
checks for epoll depth and loops that are already verified when doing
EPOLL_CTL_ADD.  This mirrors a conversion that was done for
!CONFIG_DEBUG_LOCK_ALLOC in: commit 37b5e521 ("epoll: remove
ep_call_nested() from ep_eventpoll_poll()")

Link: http://lkml.kernel.org/r/1567628549-11501-1-git-send-email-jbaron@akamai.comSigned-off-by: Jason Baron <jbaron@akamai.com>
Reviewed-by: Roman Penyaev <rpenyaev@suse.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/eventpoll.c

@@ -551,28 +551,23 @@ static int ep_call_nested(struct nested_calls *ncalls,
  */
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 
-static struct nested_calls poll_safewake_ncalls;
-
-static int ep_poll_wakeup_proc(void *priv, void *cookie, int call_nests)
-{
-	unsigned long flags;
-	wait_queue_head_t *wqueue = (wait_queue_head_t *)cookie;
-
-	spin_lock_irqsave_nested(&wqueue->lock, flags, call_nests + 1);
-	wake_up_locked_poll(wqueue, EPOLLIN);
-	spin_unlock_irqrestore(&wqueue->lock, flags);
-
-	return 0;
-}
+static DEFINE_PER_CPU(int, wakeup_nest);
 
 static void ep_poll_safewake(wait_queue_head_t *wq)
 {
-	int this_cpu = get_cpu();
-
-	ep_call_nested(&poll_safewake_ncalls,
-		       ep_poll_wakeup_proc, NULL, wq, (void *) (long) this_cpu);
+	unsigned long flags;
+	int subclass;
 
-	put_cpu();
+	local_irq_save(flags);
+	preempt_disable();
+	subclass = __this_cpu_read(wakeup_nest);
+	spin_lock_nested(&wq->lock, subclass + 1);
+	__this_cpu_inc(wakeup_nest);
+	wake_up_locked_poll(wq, POLLIN);
+	__this_cpu_dec(wakeup_nest);
+	spin_unlock(&wq->lock);
+	local_irq_restore(flags);
+	preempt_enable();
 }
 
 #else
@@ -2370,11 +2365,6 @@ static int __init eventpoll_init(void)
 	 */
 	ep_nested_calls_init(&poll_loop_ncalls);
 
-#ifdef CONFIG_DEBUG_LOCK_ALLOC
-	/* Initialize the structure used to perform safe poll wait head wake ups */
-	ep_nested_calls_init(&poll_safewake_ncalls);
-#endif
-
 	/*
 	 * We can have many thousands of epitems, so prevent this from
 	 * using an extra cache line on 64-bit (and smaller) CPUs