1 files changed, 411 insertions, 168 deletions

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index a3090b446af1..c35580194ad0 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -38,48 +38,174 @@
 #include <linux/compat.h>
 #include <linux/rculist.h>
 #include <linux/capability.h>
+#include <linux/seqlock.h>
 #include <net/busy_poll.h>
 
 /*
- * LOCKING:
- * There are three level of locking required by epoll :
+ * fs/eventpoll.c - Efficient event polling ("epoll") kernel implementation.
  *
- * 1) epnested_mutex (mutex)
- * 2) ep->mtx (mutex)
- * 3) ep->lock (spinlock)
  *
- * The acquire order is the one listed above, from 1 to 3.
- * We need a spinlock (ep->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
- * a spinlock. During the event transfer loop (from kernel to
- * user space) we could end up sleeping due a copy_to_user(), so
- * we need a lock that will allow us to sleep. This lock is a
- * mutex (ep->mtx). It is acquired during the event transfer loop,
- * during epoll_ctl(EPOLL_CTL_DEL) and during eventpoll_release_file().
- * The epnested_mutex is acquired when inserting an epoll fd onto another
- * epoll fd. We do this so that we walk the epoll tree and ensure that this
- * insertion does not create a cycle of epoll file descriptors, which
- * could lead to deadlock. We need a global mutex to prevent two
- * simultaneous inserts (A into B and B into A) from racing and
- * constructing a cycle without either insert observing that it is
- * going to.
- * It is necessary to acquire multiple "ep->mtx"es at once in the
- * case when one epoll fd is added to another. In this case, we
- * always acquire the locks in the order of nesting (i.e. after
- * epoll_ctl(e1, EPOLL_CTL_ADD, e2), e1->mtx will always be acquired
- * before e2->mtx). Since we disallow cycles of epoll file
- * descriptors, this ensures that the mutexes are well-ordered. In
- * order to communicate this nesting to lockdep, when walking a tree
- * 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 "epnested_mutex" (together with "ep->lock") to have it working,
- * but having "ep->mtx" will make the interface more scalable.
- * Events that require holding "epnested_mutex" are very rare, while for
- * normal operations the epoll private "ep->mtx" will guarantee
- * a better scalability.
+ * Overview
+ * --------
+ *
+ * Each epoll_create(2) returns an anonymous [eventpoll] file whose
+ * ->private_data is a struct eventpoll. Each EPOLL_CTL_ADD installs
+ * a struct epitem linking one (watched file, fd) pair back to that
+ * eventpoll via the watched file's f_op->poll() wait queue(s). When
+ * the watched file signals readiness, ep_poll_callback() fires and
+ * marks the epitem ready. epoll_wait(2) drains the ready list under
+ * ep->mtx, re-queueing items in level-triggered mode.
+ *
+ * epoll instances can watch other epoll instances up to EP_MAX_NESTS
+ * deep; cycles are forbidden and detected at EPOLL_CTL_ADD time.
+ *
+ *
+ * Locking
+ * -------
+ *
+ * Three levels, acquired from outer to inner:
+ *
+ *   epnested_mutex   (global; rare; taken only for EPOLL_CTL_ADD
+ *                     loop / path checks)
+ *     > ep->mtx     (per-eventpoll; sleepable; serializes most ops)
+ *       > ep->lock  (per-eventpoll; IRQ-safe spinlock)
+ *
+ *   file->f_lock    (per-file; NOT IRQ-safe; guards f_ep hlist ops;
+ *                    nested inside ep->mtx, outside ep->lock)
+ *
+ * Rationale:
+ *   - ep->lock is a spinlock because ep_poll_callback() is called from
+ *     wake_up() which may run in hard-IRQ context. All ep->lock
+ *     critical sections use spin_lock_irqsave().
+ *   - ep->mtx is a sleepable mutex because the event delivery loop
+ *     calls copy_to_user(), and ep_insert() may sleep in
+ *     kmem_cache_alloc() and f_op->poll().
+ *   - epnested_mutex is global because cycle detection needs a global
+ *     view of the epoll topology; a per-object scheme would let two
+ *     concurrent inserts (A into B, B into A) construct a cycle
+ *     without either observer seeing it.
+ *   - Per-ep ep->mtx is preferred for scalability elsewhere. Events
+ *     that require epnested_mutex are rare.
+ *
+ * When EPOLL_CTL_ADD nests one eventpoll inside another we acquire
+ * ep->mtx on both: outer first, target second. Since cycles are
+ * forbidden the set of live ep->mtx holds is always a strict chain,
+ * communicated to lockdep via mutex_lock_nested() subclasses derived
+ * from the current recursion depth.
+ *
+ *
+ * Field protection
+ * ----------------
+ *
+ * struct eventpoll:
+ *   mtx              - self
+ *   rbr              - ep->mtx
+ *   ovflist, rdllist - ep->lock (IRQ-safe)
+ *   wq               - ep->lock for queue mutation
+ *   poll_wait        - internal waitqueue spinlock
+ *   refs             - file->f_lock for adds; ep->mtx for removes;
+ *                      RCU for readers (hlist_del_rcu + kfree_rcu(ep))
+ *   ws               - ep->mtx
+ *   gen, loop_check_depth - epnested_mutex
+ *   file, user       - immutable after setup
+ *   refcount         - atomic (refcount_t)
+ *   napi_*           - READ_ONCE / WRITE_ONCE
+ *
+ * struct epitem:
+ *   rbn / rcu union  - rbn: ep->mtx (while epi is linked in ep->rbr).
+ *                      rcu: written only by kfree_rcu(epi) on the free
+ *                      path; otherwise untouched by epoll code.
+ *   rdllink, next    - ep->lock
+ *   ffd, ep          - immutable after ep_insert()
+ *   pwqlist          - ep->mtx for writes; POLLFREE clears pwq->whead
+ *                      via smp_store_release(), see below
+ *   fllink           - file->f_lock for mutation; hlist_del_rcu +
+ *                      kfree_rcu(epi) for safe RCU readers
+ *   ws               - RCU (rcu_assign_pointer /
+ *                      rcu_dereference_check(mtx))
+ *   event            - ep->mtx for writes; lockless read in
+ *                      ep_poll_callback pairs with smp_mb() in
+ *                      ep_modify()
+ *
+ *
+ * Ready-list state machine
+ * ------------------------
+ *
+ * Readiness is tracked in two lists under ep->lock:
+ *
+ *   rdllist   - doubly-linked FIFO; the "current" ready list.
+ *   ovflist   - singly-linked LIFO; used during a scan to catch
+ *               events that arrive while rdllist is being iterated
+ *               without ep->lock.
+ *
+ * Encoded in ep->ovflist:
+ *   EP_UNACTIVE_PTR - no scan active; callback appends to rdllist.
+ *   NULL            - scan active, no spill yet.
+ *   pointer to epi  - scan active with spilled items (LIFO).
+ *
+ * Encoded in epi->ovflist_next:
+ *   EP_UNACTIVE_PTR - epi is not on ovflist.
+ *   otherwise       - next epi on ovflist (NULL at tail).
+ *
+ * ep_start_scan() flips "not scanning" to "scanning" and splices
+ * rdllist into a caller-local scan_batch. ep_done_scan() drains ovflist
+ * back to rdllist (list_add head-insert reverses LIFO to FIFO),
+ * flips back to "not scanning", and re-splices any items the caller
+ * left in scan_batch (e.g., level-triggered re-queues).
+ *
+ *
+ * Removal paths
+ * -------------
+ *
+ * Three paths dispose of epitems and/or eventpolls:
+ *
+ *   A. ep_remove()              - EPOLL_CTL_DEL and ep_insert()
+ *                                 rollback. Caller holds ep->mtx.
+ *   B. ep_clear_and_put()       - close of the epoll fd itself
+ *                                 (ep_eventpoll_release).
+ *   C. eventpoll_release_file() - close of a watched file, invoked
+ *                                 from __fput().
+ *
+ * Coordination:
+ *   A and C exclude each other via the watched file's refcount.
+ *   A pins the file with epi_fget() before touching file->f_ep or
+ *   file->f_lock; if the pin fails, __fput() is in flight and C
+ *   will clean this epi up. See the epi_fget() block comment.
+ *   A and B both hold ep->mtx serially. B walks the rbtree with
+ *   rb_next() captured before ep_remove() erases the current node.
+ *   B and C both take ep->mtx; the loser sees fewer entries or an
+ *   empty file->f_ep.
+ *
+ * Within every path the internal order is strict:
+ *   ep_unregister_pollwait()  - drain pwqlist; synchronizes with any
+ *                                in-flight ep_poll_callback via the
+ *                                watched wait-queue head's lock.
+ *   ep_remove_file()          - hlist_del_rcu of epi->fllink and,
+ *                                if last watcher, clear file->f_ep,
+ *                                under file->f_lock.
+ *   ep_remove_epi()           - rb_erase, rdllist unlink (ep->lock),
+ *                                wakeup_source_unregister,
+ *                                kfree_rcu(epi).
+ *
+ * kfree_rcu(epi) defers the free past RCU readers in
+ * reverse_path_check_proc(); kfree_rcu(ep) defers past readers in
+ * ep_get_upwards_depth_proc().
+ *
+ *
+ * POLLFREE handshake
+ * ------------------
+ *
+ * When a subsystem tears down a wait-queue head that an epitem is
+ * registered on (binder, signalfd, ...), it wakes the callback with
+ * POLLFREE and must RCU-defer the head's free. The store/load pair:
+ *
+ *   ep_poll_callback() POLLFREE branch:
+ *     smp_store_release(&pwq->whead, NULL)
+ *
+ *   ep_remove_wait_queue():
+ *     smp_load_acquire(&pwq->whead)
+ *
+ * See those sites for the full argument.
  */
 
 /* Epoll private bits inside the event mask */
@@ -136,14 +262,16 @@ struct epitem {
 		struct rcu_head rcu;
 	};
 
-	/* List header used to link this structure to the eventpoll ready list */
+	/* Link on the owning eventpoll's ready list (ep->rdllist). */
 	struct list_head rdllink;
 
 	/*
-	 * Works together "struct eventpoll"->ovflist in keeping the
-	 * single linked chain of items.
+	 * Link on the owning eventpoll's scan-overflow list (ep->ovflist),
+	 * EP_UNACTIVE_PTR when not linked. See epi_on_ovflist() /
+	 * epi_clear_ovflist() and the "Ready-list state machine" section
+	 * in the top-of-file banner.
 	 */
-	struct epitem *next;
+	struct epitem *ovflist_next;
 
 	/* The file descriptor information this item refers to */
 	struct epoll_filefd ffd;
@@ -190,6 +318,9 @@ struct eventpoll {
 	/* Lock which protects rdllist and ovflist */
 	spinlock_t lock;
 
+	/* Protect switching between rdllist and ovflist */
+	seqcount_spinlock_t seq;
+
 	/* RB tree root used to store monitored fd structs */
 	struct rb_root_cached rbr;
 
@@ -372,6 +503,43 @@ static inline struct epitem *ep_item_from_wait(wait_queue_entry_t *p)
 	return container_of(p, struct eppoll_entry, wait)->base;
 }
 
+/*
+ * Ready-list / ovflist state (see "Ready-list state machine" in the
+ * top-of-file banner for the full state machine). EP_UNACTIVE_PTR is
+ * the sentinel; these wrappers name each transition and each test so
+ * call sites do not need to know the sentinel's value.
+ */
+
+/* True iff @ep is between ep_enter_scan() and ep_exit_scan(). */
+static inline bool ep_is_scanning(struct eventpoll *ep)
+{
+	return READ_ONCE(ep->ovflist) != EP_UNACTIVE_PTR;
+}
+
+/* Called by ep_start_scan(): divert ep_poll_callback() to ovflist. */
+static inline void ep_enter_scan(struct eventpoll *ep)
+{
+	WRITE_ONCE(ep->ovflist, NULL);
+}
+
+/* Called by ep_done_scan(): redirect ep_poll_callback() back to rdllist. */
+static inline void ep_exit_scan(struct eventpoll *ep)
+{
+	WRITE_ONCE(ep->ovflist, EP_UNACTIVE_PTR);
+}
+
+/* True iff @epi is currently linked on its ep's ovflist. */
+static inline bool epi_on_ovflist(const struct epitem *epi)
+{
+	return epi->ovflist_next != EP_UNACTIVE_PTR;
+}
+
+/* Mark @epi as not on any ovflist (init and post-drain). */
+static inline void epi_clear_ovflist(struct epitem *epi)
+{
+	epi->ovflist_next = EP_UNACTIVE_PTR;
+}
+
 /**
  * ep_events_available - Checks if ready events might be available.
  *
@@ -382,8 +550,10 @@ static inline struct epitem *ep_item_from_wait(wait_queue_entry_t *p)
  */
 static inline int ep_events_available(struct eventpoll *ep)
 {
-	return !list_empty_careful(&ep->rdllist) ||
-		READ_ONCE(ep->ovflist) != EP_UNACTIVE_PTR;
+	unsigned int seq = read_seqcount_begin(&ep->seq);
+
+	return !list_empty_careful(&ep->rdllist) || ep_is_scanning(ep) ||
+		read_seqcount_retry(&ep->seq, seq);
 }
 
 #ifdef CONFIG_NET_RX_BUSY_POLL
@@ -723,7 +893,7 @@ static inline void ep_pm_stay_awake_rcu(struct epitem *epi)
  * ep->mutex needs to be held because we could be hit by
  * eventpoll_release_file() and epoll_ctl().
  */
-static void ep_start_scan(struct eventpoll *ep, struct list_head *txlist)
+static void ep_start_scan(struct eventpoll *ep, struct list_head *scan_batch)
 {
 	/*
 	 * Steal the ready list, and re-init the original one to the
@@ -735,13 +905,17 @@ static void ep_start_scan(struct eventpoll *ep, struct list_head *txlist)
 	 */
 	lockdep_assert_irqs_enabled();
 	spin_lock_irq(&ep->lock);
-	list_splice_init(&ep->rdllist, txlist);
-	WRITE_ONCE(ep->ovflist, NULL);
+	write_seqcount_begin(&ep->seq);
+
+	list_splice_init(&ep->rdllist, scan_batch);
+	ep_enter_scan(ep);
+
+	write_seqcount_end(&ep->seq);
 	spin_unlock_irq(&ep->lock);
 }
 
 static void ep_done_scan(struct eventpoll *ep,
-			 struct list_head *txlist)
+			 struct list_head *scan_batch)
 {
 	struct epitem *epi, *nepi;
 
@@ -751,34 +925,35 @@ static void ep_done_scan(struct eventpoll *ep,
 	 * other events might have been queued by the poll callback.
 	 * We re-insert them inside the main ready-list here.
 	 */
-	for (nepi = READ_ONCE(ep->ovflist); (epi = nepi) != NULL;
-	     nepi = epi->next, epi->next = EP_UNACTIVE_PTR) {
+	for (nepi = READ_ONCE(ep->ovflist); (epi = nepi) != NULL; ) {
+		nepi = epi->ovflist_next;
+		epi_clear_ovflist(epi);
 		/*
-		 * We need to check if the item is already in the list.
-		 * During the "sproc" callback execution time, items are
-		 * queued into ->ovflist but the "txlist" might already
-		 * contain them, and the list_splice() below takes care of them.
+		 * Skip items that the caller already returned via @scan_batch
+		 * -- the list_splice() below takes care of those.
 		 */
 		if (!ep_is_linked(epi)) {
 			/*
-			 * ->ovflist is LIFO, so we have to reverse it in order
-			 * to keep in FIFO.
+			 * ovflist is LIFO; list_add() head-insert here
+			 * reverses the iteration order into FIFO.
 			 */
 			list_add(&epi->rdllink, &ep->rdllist);
 			ep_pm_stay_awake(epi);
 		}
 	}
-	/*
-	 * We need to set back ep->ovflist to EP_UNACTIVE_PTR, so that after
-	 * releasing the lock, events will be queued in the normal way inside
-	 * ep->rdllist.
-	 */
-	WRITE_ONCE(ep->ovflist, EP_UNACTIVE_PTR);
+
+	write_seqcount_begin(&ep->seq);
+
+	/* Back out of scan mode; callbacks target ep->rdllist again. */
+	ep_exit_scan(ep);
 
 	/*
-	 * Quickly re-inject items left on "txlist".
+	 * Quickly re-inject items left on "scan_batch".
 	 */
-	list_splice(txlist, &ep->rdllist);
+	list_splice(scan_batch, &ep->rdllist);
+
+	write_seqcount_end(&ep->seq);
+
 	__pm_relax(ep->ws);
 
 	if (!list_empty(&ep->rdllist)) {
@@ -999,7 +1174,7 @@ static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt, int depth
 static __poll_t __ep_eventpoll_poll(struct file *file, poll_table *wait, int depth)
 {
 	struct eventpoll *ep = file->private_data;
-	LIST_HEAD(txlist);
+	LIST_HEAD(scan_batch);
 	struct epitem *epi, *tmp;
 	poll_table pt;
 	__poll_t res = 0;
@@ -1014,8 +1189,8 @@ static __poll_t __ep_eventpoll_poll(struct file *file, poll_table *wait, int dep
 	 * the ready list.
 	 */
 	mutex_lock_nested(&ep->mtx, depth);
-	ep_start_scan(ep, &txlist);
-	list_for_each_entry_safe(epi, tmp, &txlist, rdllink) {
+	ep_start_scan(ep, &scan_batch);
+	list_for_each_entry_safe(epi, tmp, &scan_batch, rdllink) {
 		if (ep_item_poll(epi, &pt, depth + 1)) {
 			res = EPOLLIN | EPOLLRDNORM;
 			break;
@@ -1029,7 +1204,7 @@ static __poll_t __ep_eventpoll_poll(struct file *file, poll_table *wait, int dep
 			list_del_init(&epi->rdllink);
 		}
 	}
-	ep_done_scan(ep, &txlist);
+	ep_done_scan(ep, &scan_batch);
 	mutex_unlock(&ep->mtx);
 	return res;
 }
@@ -1155,11 +1330,12 @@ static int ep_alloc(struct eventpoll **pep)
 
 	mutex_init(&ep->mtx);
 	spin_lock_init(&ep->lock);
+	seqcount_spinlock_init(&ep->seq, &ep->lock);
 	init_waitqueue_head(&ep->wq);
 	init_waitqueue_head(&ep->poll_wait);
 	INIT_LIST_HEAD(&ep->rdllist);
 	ep->rbr = RB_ROOT_CACHED;
-	ep->ovflist = EP_UNACTIVE_PTR;
+	ep->ovflist = EP_UNACTIVE_PTR;	/* not scanning */
 	ep->user = get_current_user();
 	refcount_set(&ep->refcount, 1);
 
@@ -1283,9 +1459,9 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
 	 * semantics). All the events that happen during that period of time are
 	 * chained in ep->ovflist and requeued later on.
 	 */
-	if (READ_ONCE(ep->ovflist) != EP_UNACTIVE_PTR) {
-		if (epi->next == EP_UNACTIVE_PTR) {
-			epi->next = READ_ONCE(ep->ovflist);
+	if (ep_is_scanning(ep)) {
+		if (!epi_on_ovflist(epi)) {
+			epi->ovflist_next = READ_ONCE(ep->ovflist);
 			WRITE_ONCE(ep->ovflist, epi);
 			ep_pm_stay_awake_rcu(epi);
 		}
@@ -1526,7 +1702,7 @@ static noinline void ep_destroy_wakeup_source(struct epitem *epi)
 	wakeup_source_unregister(ws);
 }
 
-static int attach_epitem(struct file *file, struct epitem *epi)
+static int ep_attach_file(struct file *file, struct epitem *epi)
 {
 	struct epitems_head *to_free = NULL;
 	struct hlist_head *head = NULL;
@@ -1561,68 +1737,112 @@ allocate:
 }
 
 /*
- * Must be called with "mtx" held.
+ * Charge the user's epoll_watches quota, allocate a fresh epitem for
+ * @tfile/@fd, and initialize its fields. The returned item is not yet
+ * linked into any data structure; the caller must install it via
+ * ep_register_epitem() (which takes over on success) or kmem_cache_free()
+ * it and decrement epoll_watches on its own.
+ *
+ * Returns ERR_PTR(-ENOSPC) if the quota is exceeded, ERR_PTR(-ENOMEM)
+ * if the slab allocation fails.
  */
-static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
-		     struct file *tfile, int fd, int full_check)
+static struct epitem *ep_alloc_epitem(struct eventpoll *ep,
+				      const struct epoll_event *event,
+				      struct file *tfile, int fd)
 {
-	int error, pwake = 0;
-	__poll_t revents;
 	struct epitem *epi;
-	struct ep_pqueue epq;
-	struct eventpoll *tep = NULL;
-
-	if (is_file_epoll(tfile))
-		tep = tfile->private_data;
-
-	lockdep_assert_irqs_enabled();
 
 	if (unlikely(percpu_counter_compare(&ep->user->epoll_watches,
 					    max_user_watches) >= 0))
-		return -ENOSPC;
+		return ERR_PTR(-ENOSPC);
 	percpu_counter_inc(&ep->user->epoll_watches);
 
-	if (!(epi = kmem_cache_zalloc(epi_cache, GFP_KERNEL))) {
+	epi = kmem_cache_zalloc(epi_cache, GFP_KERNEL);
+	if (unlikely(!epi)) {
 		percpu_counter_dec(&ep->user->epoll_watches);
-		return -ENOMEM;
+		return ERR_PTR(-ENOMEM);
 	}
 
-	/* Item initialization follow here ... */
 	INIT_LIST_HEAD(&epi->rdllink);
 	epi->ep = ep;
 	ep_set_ffd(&epi->ffd, tfile, fd);
 	epi->event = *event;
-	epi->next = EP_UNACTIVE_PTR;
+	epi_clear_ovflist(epi);
+
+	return epi;
+}
+
+/*
+ * Install @epi into its target file's f_ep hlist and into @ep's rbtree,
+ * taking one additional reference on @ep for the lifetime of the item.
+ *
+ * If @tep is non-NULL, the target file is itself an eventpoll; we hold
+ * tep->mtx at subclass 1 across the attach + rbtree insert to serialize
+ * with the target side. RB tree ops are protected by @ep->mtx, which
+ * the caller already holds.
+ *
+ * On failure the epi is freed and the epoll_watches counter decremented,
+ * matching ep_alloc_epitem()'s allocation. After this returns
+ * successfully, ep_insert()'s later error paths use ep_remove() for
+ * unwind; that cannot drop @ep's refcount to zero because the ep file
+ * itself still holds the original reference.
+ */
+static int ep_register_epitem(struct eventpoll *ep, struct epitem *epi,
+			      struct eventpoll *tep, int full_check)
+{
+	struct file *tfile = epi->ffd.file;
+	int error;
 
 	if (tep)
 		mutex_lock_nested(&tep->mtx, 1);
-	/* Add the current item to the list of active epoll hook for this file */
-	if (unlikely(attach_epitem(tfile, epi) < 0)) {
+
+	error = ep_attach_file(tfile, epi);
+	if (unlikely(error)) {
 		if (tep)
 			mutex_unlock(&tep->mtx);
 		kmem_cache_free(epi_cache, epi);
 		percpu_counter_dec(&ep->user->epoll_watches);
-		return -ENOMEM;
+		return error;
 	}
 
 	if (full_check && !tep)
 		list_file(tfile);
 
-	/*
-	 * Add the current item to the RB tree. All RB tree operations are
-	 * protected by "mtx", and ep_insert() is called with "mtx" held.
-	 */
 	ep_rbtree_insert(ep, epi);
+
 	if (tep)
 		mutex_unlock(&tep->mtx);
 
-	/*
-	 * ep_remove() calls in the later error paths can't lead to
-	 * ep_free() as the ep file itself still holds an ep reference.
-	 */
 	ep_get(ep);
+	return 0;
+}
+
+/*
+ * Must be called with "mtx" held.
+ */
+static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
+		     struct file *tfile, int fd, int full_check)
+{
+	int error, pwake = 0;
+	__poll_t revents;
+	struct epitem *epi;
+	struct ep_pqueue epq;
+	struct eventpoll *tep = NULL;
 
-	/* now check if we've created too many backpaths */
+	if (is_file_epoll(tfile))
+		tep = tfile->private_data;
+
+	lockdep_assert_irqs_enabled();
+
+	epi = ep_alloc_epitem(ep, event, tfile, fd);
+	if (IS_ERR(epi))
+		return PTR_ERR(epi);
+
+	error = ep_register_epitem(ep, epi, tep, full_check);
+	if (error)
+		return error;
+
+	/* Reject the insert if the new link would create too many back-paths. */
 	if (unlikely(full_check && reverse_path_check())) {
 		ep_remove(ep, epi);
 		return -EINVAL;
@@ -1649,28 +1869,21 @@ static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
 	 */
 	revents = ep_item_poll(epi, &epq.pt, 1);
 
-	/*
-	 * We have to check if something went wrong during the poll wait queue
-	 * install process. Namely an allocation for a wait queue failed due
-	 * high memory pressure.
-	 */
+	/* ep_ptable_queue_proc() signals allocation failure by clearing epq.epi. */
 	if (unlikely(!epq.epi)) {
 		ep_remove(ep, epi);
 		return -ENOMEM;
 	}
 
-	/* We have to drop the new item inside our item list to keep track of it */
+	/* Drop the new item onto the ready list if it is already ready. */
 	spin_lock_irq(&ep->lock);
 
-	/* record NAPI ID of new item if present */
 	ep_set_busy_poll_napi_id(epi);
 
-	/* If the file is already "ready" we drop it inside the ready list */
 	if (revents && !ep_is_linked(epi)) {
 		list_add_tail(&epi->rdllink, &ep->rdllist);
 		ep_pm_stay_awake(epi);
 
-		/* Notify waiting tasks that events are available */
 		if (waitqueue_active(&ep->wq))
 			wake_up(&ep->wq);
 		if (waitqueue_active(&ep->poll_wait))
@@ -1762,11 +1975,87 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi,
 	return 0;
 }
 
+/*
+ * Attempt to deliver one event for @epi into @*uevents.
+ *
+ * Returns 1 if an event was delivered (with *uevents advanced to the
+ * next slot), 0 if the re-poll reported no caller-requested events
+ * (@epi drops out of the ready list; a future callback will re-add
+ * it), or -EFAULT if copy_to_user() faulted (in which case @epi is
+ * re-inserted at the head of @scan_batch so ep_done_scan() merges it
+ * back to rdllist for the next attempt).
+ *
+ * PM bookkeeping and level-triggered re-queue are handled here.
+ * Caller holds ep->mtx and the scan is active.
+ */
+static int ep_deliver_event(struct eventpoll *ep, struct epitem *epi,
+			    poll_table *pt,
+			    struct epoll_event __user **uevents,
+			    struct list_head *scan_batch)
+{
+	struct epoll_event __user *next;
+	struct wakeup_source *ws;
+	__poll_t revents;
+
+	/*
+	 * Activate ep->ws before deactivating epi->ws to prevent
+	 * triggering auto-suspend here (in case we reactivate epi->ws
+	 * below).  Rearranging to delay the deactivation would let
+	 * epi->ws drift out of sync with ep_is_linked().
+	 */
+	ws = ep_wakeup_source(epi);
+	if (ws) {
+		if (ws->active)
+			__pm_stay_awake(ep->ws);
+		__pm_relax(ws);
+	}
+
+	list_del_init(&epi->rdllink);
+
+	/*
+	 * Re-poll under ep->mtx so userspace cannot change the item
+	 * out from under us. If no caller-requested events remain,
+	 * @epi stays off the ready list; the poll callback will
+	 * re-queue it when events next appear.
+	 */
+	revents = ep_item_poll(epi, pt, 1);
+	if (!revents)
+		return 0;
+
+	next = epoll_put_uevent(revents, epi->event.data, *uevents);
+	if (!next) {
+		/*
+		 * copy_to_user() faulted: put the item back so
+		 * ep_done_scan() splices it onto rdllist for the next
+		 * attempt.
+		 */
+		list_add(&epi->rdllink, scan_batch);
+		ep_pm_stay_awake(epi);
+		return -EFAULT;
+	}
+	*uevents = next;
+
+	if (epi->event.events & EPOLLONESHOT) {
+		epi->event.events &= EP_PRIVATE_BITS;
+	} else if (!(epi->event.events & EPOLLET)) {
+		/*
+		 * Level-triggered: re-queue so the next epoll_wait()
+		 * rechecks availability. We are the sole writer to
+		 * rdllist here -- epoll_ctl() callers are locked out
+		 * by ep->mtx, and the poll callback queues to ovflist
+		 * during scans.
+		 */
+		list_add_tail(&epi->rdllink, &ep->rdllist);
+		ep_pm_stay_awake(epi);
+	}
+	return 1;
+}
+
 static int ep_send_events(struct eventpoll *ep,
 			  struct epoll_event __user *events, int maxevents)
 {
 	struct epitem *epi, *tmp;
-	LIST_HEAD(txlist);
+	LIST_HEAD(scan_batch);
 	poll_table pt;
 	int res = 0;
 
@@ -1781,74 +2070,28 @@ static int ep_send_events(struct eventpoll *ep,
 	init_poll_funcptr(&pt, NULL);
 
 	mutex_lock(&ep->mtx);
-	ep_start_scan(ep, &txlist);
+	ep_start_scan(ep, &scan_batch);
 
 	/*
-	 * We can loop without lock because we are passed a task private list.
-	 * Items cannot vanish during the loop we are holding ep->mtx.
+	 * We can loop without lock because we are passed a task-private
+	 * scan_batch; items cannot vanish while we hold ep->mtx.
 	 */
-	list_for_each_entry_safe(epi, tmp, &txlist, rdllink) {
-		struct wakeup_source *ws;
-		__poll_t revents;
+	list_for_each_entry_safe(epi, tmp, &scan_batch, rdllink) {
+		int delivered;
 
 		if (res >= maxevents)
 			break;
 
-		/*
-		 * Activate ep->ws before deactivating epi->ws to prevent
-		 * triggering auto-suspend here (in case we reactive epi->ws
-		 * below).
-		 *
-		 * This could be rearranged to delay the deactivation of epi->ws
-		 * instead, but then epi->ws would temporarily be out of sync
-		 * with ep_is_linked().
-		 */
-		ws = ep_wakeup_source(epi);
-		if (ws) {
-			if (ws->active)
-				__pm_stay_awake(ep->ws);
-			__pm_relax(ws);
-		}
-
-		list_del_init(&epi->rdllink);
-
-		/*
-		 * If the event mask intersect the caller-requested one,
-		 * deliver the event to userspace. Again, we are holding ep->mtx,
-		 * so no operations coming from userspace can change the item.
-		 */
-		revents = ep_item_poll(epi, &pt, 1);
-		if (!revents)
-			continue;
-
-		events = epoll_put_uevent(revents, epi->event.data, events);
-		if (!events) {
-			list_add(&epi->rdllink, &txlist);
-			ep_pm_stay_awake(epi);
+		delivered = ep_deliver_event(ep, epi, &pt, &events, &scan_batch);
+		if (delivered < 0) {
 			if (!res)
-				res = -EFAULT;
+				res = delivered;
 			break;
 		}
-		res++;
-		if (epi->event.events & EPOLLONESHOT)
-			epi->event.events &= EP_PRIVATE_BITS;
-		else if (!(epi->event.events & EPOLLET)) {
-			/*
-			 * If this file has been added with Level
-			 * Trigger mode, we need to insert back inside
-			 * the ready list, so that the next call to
-			 * epoll_wait() will check again the events
-			 * availability. At this point, no one can insert
-			 * into ep->rdllist besides us. The epoll_ctl()
-			 * callers are locked out by
-			 * ep_send_events() holding "mtx" and the
-			 * poll callback will queue them in ep->ovflist.
-			 */
-			list_add_tail(&epi->rdllink, &ep->rdllist);
-			ep_pm_stay_awake(epi);
-		}
+		res += delivered;
 	}
-	ep_done_scan(ep, &txlist);
+
+	ep_done_scan(ep, &scan_batch);
 	mutex_unlock(&ep->mtx);
 
 	return res;