epoll: use the waitqueue lock to protect ep->wq

Patch series "waitqueue lockdep annotation", v3.

This series adds a strategic lockdep_assert_held to __wake_up_common to
ensure callers really do hold the wait_queue_head lock when calling the
unlocked wake_up variants.  It turns out epoll did not do this for a
fairly common path (hit all the time by systemd during bootup), so the
second patch fixed this instance as well.

This patch (of 3):

The epoll code currently uses the unlocked waitqueue helpers for managing
ep->wq, but instead of holding the waitqueue lock around these calls, it
uses its own ep->lock spinlock.  Given that the waitqueue is not exposed
to the rest of the kernel this actually works ok at the moment, but
prevents the epoll locking rules from being enforced using lockdep.
Remove ep->lock and use the waitqueue lock to not only reduce the size of
struct eventpoll but also to make sure we can assert locking invariants in
the waitqueue code.

Link: http://lkml.kernel.org/r/20171214152344.6880-2-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Baron <jbaron@akamai.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/eventpoll.c

@@ -50,10 +50,10 @@
  *
  * 1) epmutex (mutex)
  * 2) ep->mtx (mutex)
- * 3) ep->lock (spinlock)
+ * 3) ep->wq.lock (spinlock)
  *
  * The acquire order is the one listed above, from 1 to 3.
- * We need a spinlock (ep->lock) because we manipulate objects
+ * We need a spinlock (ep->wq.lock) because we manipulate objects
  * from inside the poll callback, that might be triggered from
  * a wake_up() that in turn might be called from IRQ context.
  * So we can't sleep inside the poll callback and hence we need
@@ -85,7 +85,7 @@
  * of epoll file descriptors, we use the current recursion depth as
  * the lockdep subkey.
  * It is possible to drop the "ep->mtx" and to use the global
- * mutex "epmutex" (together with "ep->lock") to have it working,
+ * mutex "epmutex" (together with "ep->wq.lock") to have it working,
  * but having "ep->mtx" will make the interface more scalable.
  * Events that require holding "epmutex" are very rare, while for
  * normal operations the epoll private "ep->mtx" will guarantee
@@ -182,11 +182,10 @@ struct epitem {
  * This structure is stored inside the "private_data" member of the file
  * structure and represents the main data structure for the eventpoll
  * interface.
+ *
+ * Access to it is protected by the lock inside wq.
  */
 struct eventpoll {
-	/* Protect the access to this structure */
-	spinlock_t lock;
-
 	/*
 	 * This mutex is used to ensure that files are not removed
 	 * while epoll is using them. This is held during the event
@@ -210,7 +209,7 @@ struct eventpoll {
 	/*
 	 * This is a single linked list that chains all the "struct epitem" that
 	 * happened while transferring ready events to userspace w/out
-	 * holding ->lock.
+	 * holding ->wq.lock.
 	 */
 	struct epitem *ovflist;
 
@@ -688,17 +687,17 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
 	 * because we want the "sproc" callback to be able to do it
 	 * in a lockless way.
 	 */
-	spin_lock_irqsave(&ep->lock, flags);
+	spin_lock_irqsave(&ep->wq.lock, flags);
 	list_splice_init(&ep->rdllist, &txlist);
 	ep->ovflist = NULL;
-	spin_unlock_irqrestore(&ep->lock, flags);
+	spin_unlock_irqrestore(&ep->wq.lock, flags);
 
 	/*
 	 * Now call the callback function.
 	 */
 	res = (*sproc)(ep, &txlist, priv);
 
-	spin_lock_irqsave(&ep->lock, flags);
+	spin_lock_irqsave(&ep->wq.lock, flags);
 	/*
 	 * During the time we spent inside the "sproc" callback, some
 	 * other events might have been queued by the poll callback.
@@ -740,7 +739,7 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
 		if (waitqueue_active(&ep->poll_wait))
 			pwake++;
 	}
-	spin_unlock_irqrestore(&ep->lock, flags);
+	spin_unlock_irqrestore(&ep->wq.lock, flags);
 
 	if (!ep_locked)
 		mutex_unlock(&ep->mtx);
@@ -768,12 +767,7 @@ static int ep_remove(struct eventpoll *ep, struct epitem *epi)
 	struct file *file = epi->ffd.file;
 
 	/*
-	 * Removes poll wait queue hooks. We _have_ to do this without holding
-	 * the "ep->lock" otherwise a deadlock might occur. This because of the
-	 * sequence of the lock acquisition. Here we do "ep->lock" then the wait
-	 * queue head lock when unregistering the wait queue. The wakeup callback
-	 * will run by holding the wait queue head lock and will call our callback
-	 * that will try to get "ep->lock".
+	 * Removes poll wait queue hooks.
 	 */
 	ep_unregister_pollwait(ep, epi);
 
@@ -784,10 +778,10 @@ static int ep_remove(struct eventpoll *ep, struct epitem *epi)
 
 	rb_erase_cached(&epi->rbn, &ep->rbr);
 
-	spin_lock_irqsave(&ep->lock, flags);
+	spin_lock_irqsave(&ep->wq.lock, flags);
 	if (ep_is_linked(&epi->rdllink))
 		list_del_init(&epi->rdllink);
-	spin_unlock_irqrestore(&ep->lock, flags);
+	spin_unlock_irqrestore(&ep->wq.lock, flags);
 
 	wakeup_source_unregister(ep_wakeup_source(epi));
 	/*
@@ -837,7 +831,7 @@ static void ep_free(struct eventpoll *ep)
 	 * Walks through the whole tree by freeing each "struct epitem". At this
 	 * point we are sure no poll callbacks will be lingering around, and also by
 	 * holding "epmutex" we can be sure that no file cleanup code will hit
-	 * us during this operation. So we can avoid the lock on "ep->lock".
+	 * us during this operation. So we can avoid the lock on "ep->wq.lock".
 	 * We do not need to lock ep->mtx, either, we only do it to prevent
 	 * a lockdep warning.
 	 */
@@ -1017,7 +1011,6 @@ static int ep_alloc(struct eventpoll **pep)
 	if (unlikely(!ep))
 		goto free_uid;
 
-	spin_lock_init(&ep->lock);
 	mutex_init(&ep->mtx);
 	init_waitqueue_head(&ep->wq);
 	init_waitqueue_head(&ep->poll_wait);
@@ -1122,7 +1115,7 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
 	__poll_t pollflags = key_to_poll(key);
 	int ewake = 0;
 
-	spin_lock_irqsave(&ep->lock, flags);
+	spin_lock_irqsave(&ep->wq.lock, flags);
 
 	ep_set_busy_poll_napi_id(epi);
 
@@ -1199,7 +1192,7 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
 		pwake++;
 
 out_unlock:
-	spin_unlock_irqrestore(&ep->lock, flags);
+	spin_unlock_irqrestore(&ep->wq.lock, flags);
 
 	/* We have to call this outside the lock */
 	if (pwake)
@@ -1484,7 +1477,7 @@ static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
 		goto error_remove_epi;
 
 	/* We have to drop the new item inside our item list to keep track of it */
-	spin_lock_irqsave(&ep->lock, flags);
+	spin_lock_irqsave(&ep->wq.lock, flags);
 
 	/* record NAPI ID of new item if present */
 	ep_set_busy_poll_napi_id(epi);
@@ -1501,7 +1494,7 @@ static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
 			pwake++;
 	}
 
-	spin_unlock_irqrestore(&ep->lock, flags);
+	spin_unlock_irqrestore(&ep->wq.lock, flags);
 
 	atomic_long_inc(&ep->user->epoll_watches);
 
@@ -1527,10 +1520,10 @@ error_unregister:
 	 * list, since that is used/cleaned only inside a section bound by "mtx".
 	 * And ep_insert() is called with "mtx" held.
 	 */
-	spin_lock_irqsave(&ep->lock, flags);
+	spin_lock_irqsave(&ep->wq.lock, flags);
 	if (ep_is_linked(&epi->rdllink))
 		list_del_init(&epi->rdllink);
-	spin_unlock_irqrestore(&ep->lock, flags);
+	spin_unlock_irqrestore(&ep->wq.lock, flags);
 
 	wakeup_source_unregister(ep_wakeup_source(epi));
 
@@ -1572,9 +1565,9 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi,
 	 * 1) Flush epi changes above to other CPUs.  This ensures
 	 *    we do not miss events from ep_poll_callback if an
 	 *    event occurs immediately after we call f_op->poll().
-	 *    We need this because we did not take ep->lock while
+	 *    We need this because we did not take ep->wq.lock while
 	 *    changing epi above (but ep_poll_callback does take
-	 *    ep->lock).
+	 *    ep->wq.lock).
 	 *
 	 * 2) We also need to ensure we do not miss _past_ events
 	 *    when calling f_op->poll().  This barrier also
@@ -1593,7 +1586,7 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi,
 	 * list, push it inside.
 	 */
 	if (ep_item_poll(epi, &pt, 1)) {
-		spin_lock_irq(&ep->lock);
+		spin_lock_irq(&ep->wq.lock);
 		if (!ep_is_linked(&epi->rdllink)) {
 			list_add_tail(&epi->rdllink, &ep->rdllist);
 			ep_pm_stay_awake(epi);
@@ -1604,7 +1597,7 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi,
 			if (waitqueue_active(&ep->poll_wait))
 				pwake++;
 		}
-		spin_unlock_irq(&ep->lock);
+		spin_unlock_irq(&ep->wq.lock);
 	}
 
 	/* We have to call this outside the lock */
@@ -1756,7 +1749,7 @@ 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->lock, flags);
+		spin_lock_irqsave(&ep->wq.lock, flags);
 		goto check_events;
 	}
 
@@ -1765,7 +1758,7 @@ fetch_events:
 	if (!ep_events_available(ep))
 		ep_busy_loop(ep, timed_out);
 
-	spin_lock_irqsave(&ep->lock, flags);
+	spin_lock_irqsave(&ep->wq.lock, flags);
 
 	if (!ep_events_available(ep)) {
 		/*
@@ -1807,11 +1800,11 @@ fetch_events:
 				break;
 			}
 
-			spin_unlock_irqrestore(&ep->lock, flags);
+			spin_unlock_irqrestore(&ep->wq.lock, flags);
 			if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS))
 				timed_out = 1;
 
-			spin_lock_irqsave(&ep->lock, flags);
+			spin_lock_irqsave(&ep->wq.lock, flags);
 		}
 
 		__remove_wait_queue(&ep->wq, &wait);
@@ -1821,7 +1814,7 @@ check_events:
 	/* Is it worth to try to dig for events ? */
 	eavail = ep_events_available(ep);
 
-	spin_unlock_irqrestore(&ep->lock, flags);
+	spin_unlock_irqrestore(&ep->wq.lock, flags);
 
 	/*
 	 * Try to transfer events to user space. In case we get 0 events and