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

[Jes Sorensen]

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 <jsoren...@fb.com>
Reviewed-by: Josef Bacik <jba...@fb.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