linux-kernel - [PATCH 2/5] inotify: Use mutex to prevent threaded clients reading events out of order

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Message-Id: <20170324213638.9114-3-jsorensen@fb.com>
Date:   Fri, 24 Mar 2017 17:36:35 -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 2/5] inotify: Use mutex to prevent threaded clients reading events out of order

Introduces mutex in the read() path to prevent a threaded client reading
from the same fd consuming events out of order.

This will matter when avoiding taking the spinlock when consuming each
event in the read() path.

Signed-off-by: Jes Sorensen <jsorensen@...com>
Reviewed-by: Josef Bacik <jbacik@...com>
---
 fs/notify/inotify/inotify_fsnotify.c | 1 +
 fs/notify/inotify/inotify_user.c     | 4 ++++
 include/linux/fsnotify_backend.h     | 3 +++
 3 files changed, 8 insertions(+)

diff --git a/fs/notify/inotify/inotify_fsnotify.c b/fs/notify/inotify/inotify_fsnotify.c
index 1aeb837..63c071f 100644
--- a/fs/notify/inotify/inotify_fsnotify.c
+++ b/fs/notify/inotify/inotify_fsnotify.c
@@ -168,6 +168,7 @@ static void inotify_free_group_priv(struct fsnotify_group *group)
 	idr_destroy(&group->inotify_data.idr);
 	if (group->inotify_data.ucounts)
 		dec_inotify_instances(group->inotify_data.ucounts);
+	mutex_destroy(&group->inotify_data.consumer_mutex);
 }
 
 static void inotify_free_event(struct fsnotify_event *fsn_event)
diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
index 498d609..6f5b360 100644
--- a/fs/notify/inotify/inotify_user.c
+++ b/fs/notify/inotify/inotify_user.c
@@ -230,6 +230,7 @@ static ssize_t inotify_read(struct file *file, char __user *buf,
 	start = buf;
 	group = file->private_data;
 
+	mutex_lock(&group->inotify_data.consumer_mutex);
 	add_wait_queue(&group->notification_waitq, &wait);
 	while (1) {
 		spin_lock(&group->notification_lock);
@@ -264,6 +265,7 @@ static ssize_t inotify_read(struct file *file, char __user *buf,
 		wait_woken(&wait, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
 	}
 	remove_wait_queue(&group->notification_waitq, &wait);
+	mutex_unlock(&group->inotify_data.consumer_mutex);
 
 	if (start != buf && ret != -EFAULT)
 		ret = buf - start;
@@ -650,6 +652,8 @@ static struct fsnotify_group *inotify_new_group(unsigned int max_events)
 
 	group->max_events = max_events;
 
+	mutex_init(&group->inotify_data.consumer_mutex);
+
 	spin_lock_init(&group->inotify_data.idr_lock);
 	idr_init(&group->inotify_data.idr);
 	group->inotify_data.ucounts = inc_ucount(current_user_ns(),
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index e6e689b..e0686ed 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -172,6 +172,9 @@ struct fsnotify_group {
 			spinlock_t	idr_lock;
 			struct idr      idr;
 			struct ucounts *ucounts;
+			struct mutex	consumer_mutex; /* Prevent out of order
+							 * delivery of events
+							 * to threaded consumers */
 		} inotify_data;
 #endif
 #ifdef CONFIG_FANOTIFY
-- 
2.9.3