fsnotify: Provide framework for dropping SRCU lock in ->handle_event

fanotify wants to drop fsnotify_mark_srcu lock when waiting for response
from userspace so that the whole notification subsystem is not blocked
during that time. This patch provides a framework for safely getting
mark reference for a mark found in the object list which pins the mark
in that list. We can then drop fsnotify_mark_srcu, wait for userspace
response and then safely continue iteration of the object list once we
reaquire fsnotify_mark_srcu.

Reviewed-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

4 files changed, 94 insertions, 0 deletions

diff --git a/fs/notify/fsnotify.h b/fs/notify/fsnotify.h
index 2a92dc06198c..86383c7865c0 100644
--- a/fs/notify/fsnotify.h
+++ b/fs/notify/fsnotify.h
@@ -8,6 +8,12 @@
 
 #include "../mount.h"
 
+struct fsnotify_iter_info {
+        struct fsnotify_mark *inode_mark;
+        struct fsnotify_mark *vfsmount_mark;
+        int srcu_idx;
+};
+
 /* destroy all events sitting in this groups notification queue */
 extern void fsnotify_flush_notify(struct fsnotify_group *group);
 
diff --git a/fs/notify/group.c b/fs/notify/group.c
index 0fb4aadcc19f..79439cdf16e0 100644
--- a/fs/notify/group.c
+++ b/fs/notify/group.c
@@ -126,6 +126,7 @@ struct fsnotify_group *fsnotify_alloc_group(const struct fsnotify_ops *ops)
         /* set to 0 when there a no external references to this group */
         atomic_set(&group->refcnt, 1);
         atomic_set(&group->num_marks, 0);
+        atomic_set(&group->user_waits, 0);
 
         spin_lock_init(&group->notification_lock);
         INIT_LIST_HEAD(&group->notification_list);
diff --git a/fs/notify/mark.c b/fs/notify/mark.c
index f916b71c9139..c4f43a6acd9a 100644
--- a/fs/notify/mark.c
+++ b/fs/notify/mark.c
@@ -109,6 +109,16 @@ void fsnotify_get_mark(struct fsnotify_mark *mark)
         atomic_inc(&mark->refcnt);
 }
 
+/*
+ * Get mark reference when we found the mark via lockless traversal of object
+ * list. Mark can be already removed from the list by now and on its way to be
+ * destroyed once SRCU period ends.
+ */
+static bool fsnotify_get_mark_safe(struct fsnotify_mark *mark)
+{
+        return atomic_inc_not_zero(&mark->refcnt);
+}
+
 static void __fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
 {
         u32 new_mask = 0;
@@ -243,6 +253,72 @@ void fsnotify_put_mark(struct fsnotify_mark *mark)
                            FSNOTIFY_REAPER_DELAY);
 }
 
+bool fsnotify_prepare_user_wait(struct fsnotify_iter_info *iter_info)
+{
+        struct fsnotify_group *group;
+
+        if (WARN_ON_ONCE(!iter_info->inode_mark && !iter_info->vfsmount_mark))
+                return false;
+
+        if (iter_info->inode_mark)
+                group = iter_info->inode_mark->group;
+        else
+                group = iter_info->vfsmount_mark->group;
+
+        /*
+         * Since acquisition of mark reference is an atomic op as well, we can
+         * be sure this inc is seen before any effect of refcount increment.
+         */
+        atomic_inc(&group->user_waits);
+
+        if (iter_info->inode_mark) {
+                /* This can fail if mark is being removed */
+                if (!fsnotify_get_mark_safe(iter_info->inode_mark))
+                        goto out_wait;
+        }
+        if (iter_info->vfsmount_mark) {
+                if (!fsnotify_get_mark_safe(iter_info->vfsmount_mark))
+                        goto out_inode;
+        }
+
+        /*
+         * Now that both marks are pinned by refcount in the inode / vfsmount
+         * lists, we can drop SRCU lock, and safely resume the list iteration
+         * once userspace returns.
+         */
+        srcu_read_unlock(&fsnotify_mark_srcu, iter_info->srcu_idx);
+
+        return true;
+out_inode:
+        if (iter_info->inode_mark)
+                fsnotify_put_mark(iter_info->inode_mark);
+out_wait:
+        if (atomic_dec_and_test(&group->user_waits) && group->shutdown)
+                wake_up(&group->notification_waitq);
+        return false;
+}
+
+void fsnotify_finish_user_wait(struct fsnotify_iter_info *iter_info)
+{
+        struct fsnotify_group *group = NULL;
+
+        iter_info->srcu_idx = srcu_read_lock(&fsnotify_mark_srcu);
+        if (iter_info->inode_mark) {
+                group = iter_info->inode_mark->group;
+                fsnotify_put_mark(iter_info->inode_mark);
+        }
+        if (iter_info->vfsmount_mark) {
+                group = iter_info->vfsmount_mark->group;
+                fsnotify_put_mark(iter_info->vfsmount_mark);
+        }
+        /*
+         * We abuse notification_waitq on group shutdown for waiting for all
+         * marks pinned when waiting for userspace.
+         */
+        if (atomic_dec_and_test(&group->user_waits) && group->shutdown)
+                wake_up(&group->notification_waitq);
+}
+
 /*
  * Mark mark as detached, remove it from group list. Mark still stays in object
  * list until its last reference is dropped. Note that we rely on mark being
@@ -647,6 +723,12 @@ void fsnotify_detach_group_marks(struct fsnotify_group *group)
                 fsnotify_free_mark(mark);
                 fsnotify_put_mark(mark);
         }
+        /*
+         * Some marks can still be pinned when waiting for response from
+         * userspace. Wait for those now. fsnotify_prepare_user_wait() will
+         * not succeed now so this wait is race-free.
+         */
+        wait_event(group->notification_waitq, !atomic_read(&group->user_waits));
 }
 
 /* Destroy all marks attached to inode / vfsmount */
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index a483614b25d0..5bb6d988b9f6 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -80,6 +80,7 @@ struct fsnotify_event;
 struct fsnotify_mark;
 struct fsnotify_event_private_data;
 struct fsnotify_fname;
+struct fsnotify_iter_info;
 
 /*
  * Each group much define these ops.  The fsnotify infrastructure will call
@@ -163,6 +164,8 @@ struct fsnotify_group {
         struct fsnotify_event *overflow_event;  /* Event we queue when the
                                                  * notification list is too
                                                  * full */
+        atomic_t user_waits;            /* Number of tasks waiting for user
+                                         * response */
 
         /* groups can define private fields here or use the void *private */
         union {
@@ -368,6 +371,8 @@ extern void fsnotify_clear_marks_by_group_flags(struct fsnotify_group *group, un
 extern void fsnotify_get_mark(struct fsnotify_mark *mark);
 extern void fsnotify_put_mark(struct fsnotify_mark *mark);
 extern void fsnotify_unmount_inodes(struct super_block *sb);
+extern void fsnotify_finish_user_wait(struct fsnotify_iter_info *iter_info);
+extern bool fsnotify_prepare_user_wait(struct fsnotify_iter_info *iter_info);
 
 /* put here because inotify does some weird stuff when destroying watches */
 extern void fsnotify_init_event(struct fsnotify_event *event,