linux-kernel - [PATCH 5/5] inotify: Avoid taking spinlock for each event processed in read()

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Message-Id: <20170324213638.9114-6-jsorensen@fb.com>
Date:   Fri, 24 Mar 2017 17:36:38 -0400
From:   Jes Sorensen <jes.sorensen@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     john@...nmccutchan.com, rlove@...ve.org, eparis@...isplace.org,
        josef@...icpanda.com, kernel-team@...com,
        Jes Sorensen <jsorensen@...com>
Subject: [PATCH 5/5] inotify: Avoid taking spinlock for each event processed in read()

Splice off the notification list while processing read events, which allows
it to be processed without taking and releasing the spinlock for each
event.

Signed-off-by: Jes Sorensen <jsorensen@...com>
Reviewed-by: Josef Bacik <jbacik@...com>
---
 fs/notify/inotify/inotify_user.c | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
index 4a93750..45b1f69 100644
--- a/fs/notify/inotify/inotify_user.c
+++ b/fs/notify/inotify/inotify_user.c
@@ -224,22 +224,27 @@ static ssize_t inotify_read(struct file *file, char __user *buf,
 	struct fsnotify_group *group;
 	struct fsnotify_event *kevent;
 	char __user *start;
-	int ret;
+	int ret, consumed = 0;
 	DEFINE_WAIT_FUNC(wait, woken_wake_function);
+	LIST_HEAD(tmp_list);
 
 	start = buf;
 	group = file->private_data;
 
 	mutex_lock(&group->inotify_data.consumer_mutex);
 	add_wait_queue(&group->notification_waitq, &wait);
+
+	spin_lock(&group->notification_lock);
+	list_splice_init(&group->notification_list, &tmp_list);
+	spin_unlock(&group->notification_lock);
+
 	while (1) {
-		spin_lock(&group->notification_lock);
-		kevent = get_one_event(&group->notification_list, count);
-		spin_unlock(&group->notification_lock);
+		kevent = get_one_event(&tmp_list, count);
 
 		pr_debug("%s: group=%p kevent=%p\n", __func__, group, kevent);
 
 		if (kevent) {
+			consumed++;
 			ret = PTR_ERR(kevent);
 			if (IS_ERR(kevent))
 				break;
@@ -262,8 +267,23 @@ static ssize_t inotify_read(struct file *file, char __user *buf,
 		if (start != buf)
 			break;
 
+		spin_lock(&group->notification_lock);
+		list_splice_init(&tmp_list, &group->notification_list);
+		group->q_len -= consumed;
+		consumed = 0;
+		spin_unlock(&group->notification_lock);
+
 		wait_woken(&wait, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
+
+		spin_lock(&group->notification_lock);
+		list_splice_init(&group->notification_list, &tmp_list);
+		spin_unlock(&group->notification_lock);
 	}
+	spin_lock(&group->notification_lock);
+	list_splice(&tmp_list, &group->notification_list);
+	group->q_len -= consumed;
+	spin_unlock(&group->notification_lock);
+
 	remove_wait_queue(&group->notification_waitq, &wait);
 	mutex_unlock(&group->inotify_data.consumer_mutex);
 
-- 
2.9.3