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From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	Marc Zyngier <maz@kernel.org>
Subject: [RFC PATCH 18/27] lift locking/unlocking ep->mtx out of ep_{start,done}_scan()
Date: Sun,  4 Oct 2020 03:39:20 +0100	[thread overview]
Message-ID: <20201004023929.2740074-18-viro@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20201004023929.2740074-1-viro@ZenIV.linux.org.uk>

From: Al Viro <viro@zeniv.linux.org.uk>

get rid of depth/ep_locked arguments there and document
the kludge in ep_item_poll() that has lead to ep_locked existence in
the first place

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
 fs/eventpoll.c | 57 ++++++++++++++++++++++++++-------------------------------
 1 file changed, 26 insertions(+), 31 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index ac996b959e94..f9c567af1f5f 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -554,20 +554,13 @@ static inline void ep_pm_stay_awake_rcu(struct epitem *epi)
 	rcu_read_unlock();
 }
 
-static void ep_start_scan(struct eventpoll *ep,
-			  int depth, bool ep_locked,
-			  struct list_head *txlist)
-{
-	lockdep_assert_irqs_enabled();
-
-	/*
-	 * We need to lock this because we could be hit by
-	 * eventpoll_release_file() and epoll_ctl().
-	 */
-
-	if (!ep_locked)
-		mutex_lock_nested(&ep->mtx, depth);
 
+/*
+ * 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)
+{
 	/*
 	 * Steal the ready list, and re-init the original one to the
 	 * empty list. Also, set ep->ovflist to NULL so that events
@@ -576,6 +569,7 @@ static void ep_start_scan(struct eventpoll *ep,
 	 * because we want the "sproc" callback to be able to do it
 	 * in a lockless way.
 	 */
+	lockdep_assert_irqs_enabled();
 	write_lock_irq(&ep->lock);
 	list_splice_init(&ep->rdllist, txlist);
 	WRITE_ONCE(ep->ovflist, NULL);
@@ -583,7 +577,6 @@ static void ep_start_scan(struct eventpoll *ep,
 }
 
 static void ep_done_scan(struct eventpoll *ep,
-			 int depth, bool ep_locked,
 			 struct list_head *txlist)
 {
 	struct epitem *epi, *nepi;
@@ -624,9 +617,6 @@ static void ep_done_scan(struct eventpoll *ep,
 	list_splice(txlist, &ep->rdllist);
 	__pm_relax(ep->ws);
 	write_unlock_irq(&ep->lock);
-
-	if (!ep_locked)
-		mutex_unlock(&ep->mtx);
 }
 
 static void epi_rcu_free(struct rcu_head *head)
@@ -763,11 +753,16 @@ static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt,
 
 	ep = epi->ffd.file->private_data;
 	poll_wait(epi->ffd.file, &ep->poll_wait, pt);
-	locked = pt && (pt->_qproc == ep_ptable_queue_proc);
 
-	ep_start_scan(ep, depth, locked, &txlist);
+	// kludge: ep_insert() calls us with ep->mtx already locked
+	locked = pt && (pt->_qproc == ep_ptable_queue_proc);
+	if (!locked)
+		mutex_lock_nested(&ep->mtx, depth);
+	ep_start_scan(ep, &txlist);
 	res = ep_read_events_proc(ep, &txlist, depth + 1);
-	ep_done_scan(ep, depth, locked, &txlist);
+	ep_done_scan(ep, &txlist);
+	if (!locked)
+		mutex_unlock(&ep->mtx);
 	return res & epi->event.events;
 }
 
@@ -809,9 +804,11 @@ static __poll_t ep_eventpoll_poll(struct file *file, poll_table *wait)
 	 * Proceed to find out if wanted events are really available inside
 	 * the ready list.
 	 */
-	ep_start_scan(ep, 0, false, &txlist);
+	mutex_lock(&ep->mtx);
+	ep_start_scan(ep, &txlist);
 	res = ep_read_events_proc(ep, &txlist, 1);
-	ep_done_scan(ep, 0, false, &txlist);
+	ep_done_scan(ep, &txlist);
+	mutex_unlock(&ep->mtx);
 	return res;
 }
 
@@ -1573,15 +1570,13 @@ static int ep_send_events(struct eventpoll *ep,
 
 	init_poll_funcptr(&pt, NULL);
 
-	ep_start_scan(ep, 0, false, &txlist);
+	mutex_lock(&ep->mtx);
+	ep_start_scan(ep, &txlist);
 
 	/*
 	 * We can loop without lock because we are passed a task private list.
-	 * Items cannot vanish during the loop because ep_scan_ready_list() is
-	 * holding "mtx" during this call.
+	 * Items cannot vanish during the loop we are holding ep->mtx.
 	 */
-	lockdep_assert_held(&ep->mtx);
-
 	list_for_each_entry_safe(epi, tmp, &txlist, rdllink) {
 		struct wakeup_source *ws;
 		__poll_t revents;
@@ -1609,9 +1604,8 @@ static int ep_send_events(struct eventpoll *ep,
 
 		/*
 		 * If the event mask intersect the caller-requested one,
-		 * deliver the event to userspace. Again, ep_scan_ready_list()
-		 * is holding ep->mtx, so no operations coming from userspace
-		 * can change the item.
+		 * 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)
@@ -1645,7 +1639,8 @@ static int ep_send_events(struct eventpoll *ep,
 			ep_pm_stay_awake(epi);
 		}
 	}
-	ep_done_scan(ep, 0, false, &txlist);
+	ep_done_scan(ep, &txlist);
+	mutex_unlock(&ep->mtx);
 
 	return res;
 }
-- 
2.11.0


  parent reply	other threads:[~2020-10-04  2:39 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-04  2:36 [RFC][PATCHSET] epoll cleanups Al Viro
2020-10-04  2:39 ` [RFC PATCH 01/27] epoll: switch epitem->pwqlist to single-linked list Al Viro
2020-10-04  2:39   ` [RFC PATCH 02/27] epoll: get rid of epitem->nwait Al Viro
2020-10-04  2:39   ` [RFC PATCH 03/27] untangling ep_call_nested(): get rid of useless arguments Al Viro
2020-10-04  2:39   ` [RFC PATCH 04/27] untangling ep_call_nested(): it's all serialized on epmutex Al Viro
2020-10-04  2:39   ` [RFC PATCH 05/27] untangling ep_call_nested(): take pushing cookie into a helper Al Viro
2020-10-04  2:39   ` [RFC PATCH 06/27] untangling ep_call_nested(): move push/pop of cookie into the callbacks Al Viro
2020-10-04  2:39   ` [RFC PATCH 07/27] untangling ep_call_nested(): and there was much rejoicing Al Viro
2020-10-04  2:39   ` [RFC PATCH 08/27] reverse_path_check_proc(): sane arguments Al Viro
2020-10-04  2:39   ` [RFC PATCH 09/27] reverse_path_check_proc(): don't bother with cookies Al Viro
2020-10-04  2:39   ` [RFC PATCH 10/27] clean reverse_path_check_proc() a bit Al Viro
2020-10-04  2:39   ` [RFC PATCH 11/27] ep_loop_check_proc(): lift pushing the cookie into callers Al Viro
2020-10-04  2:39   ` [RFC PATCH 12/27] get rid of ep_push_nested() Al Viro
2020-10-04  2:39   ` [RFC PATCH 13/27] ep_loop_check_proc(): saner calling conventions Al Viro
2020-10-04  2:39   ` [RFC PATCH 14/27] ep_scan_ready_list(): prepare to splitup Al Viro
2020-10-04  2:39   ` [RFC PATCH 15/27] lift the calls of ep_read_events_proc() into the callers Al Viro
2020-10-04  2:39   ` [RFC PATCH 16/27] lift the calls of ep_send_events_proc() " Al Viro
2020-10-04  2:39   ` [RFC PATCH 17/27] ep_send_events_proc(): fold into the caller Al Viro
2020-10-04  2:39   ` Al Viro [this message]
2020-10-04  2:39   ` [RFC PATCH 19/27] ep_insert(): don't open-code ep_remove() on failure exits Al Viro
2020-10-04  2:39   ` [RFC PATCH 20/27] ep_insert(): we only need tep->mtx around the insertion itself Al Viro
2020-10-04 12:56     ` [ep_insert()] 9ee1cc5666: WARNING:possible_recursive_locking_detected kernel test robot
2020-10-04 14:17       ` Al Viro
2020-10-04 14:27         ` Al Viro
2020-10-04  2:39   ` [RFC PATCH 21/27] take the common part of ep_eventpoll_poll() and ep_item_poll() into helper Al Viro
2020-10-04  2:39   ` [RFC PATCH 22/27] fold ep_read_events_proc() into the only caller Al Viro
2020-10-04  2:39   ` [RFC PATCH 23/27] ep_insert(): move creation of wakeup source past the fl_ep_links insertion Al Viro
2020-10-04  2:39   ` [RFC PATCH 24/27] convert ->f_ep_links/->fllink to hlist Al Viro
2020-10-04  2:39   ` [RFC PATCH 25/27] lift rcu_read_lock() into reverse_path_check() Al Viro
2020-10-04  2:39   ` [RFC PATCH 26/27] epoll: massage the check list insertion Al Viro
2020-10-04  2:39   ` [RFC PATCH 27/27] epoll: take epitem list out of struct file Al Viro
2020-10-05 20:37     ` Qian Cai
2020-10-05 20:49       ` Al Viro
2020-10-04  2:49 ` [RFC][PATCHSET] epoll cleanups Al Viro
2020-10-04 12:13 ` Matthew Wilcox
2020-10-04 14:15   ` Al Viro
2020-10-04 18:08 ` Linus Torvalds
2020-10-04 20:05   ` Al Viro

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