From: Jan Kara <jack@xxxxxxx>
commit abc77577a669f424c5d0c185b9994f2621c52aa4 upstream.
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@xxxxxxxxxx>
Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
[mruffell: backport: realign file fs/notify/mark.c]
Signed-off-by: Matthew Ruffell <matthew.ruffell@xxxxxxxxxxxxx>
---
fs/notify/fsnotify.h | 6 +++
fs/notify/group.c | 1 +
fs/notify/mark.c | 83 +++++++++++++++++++++++++++++++-
include/linux/fsnotify_backend.h | 5 ++
4 files changed, 94 insertions(+), 1 deletion(-)
diff --git a/fs/notify/fsnotify.h b/fs/notify/fsnotify.h
index 0a3bc2cf192c..0ad0eb9f2e14 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 fbe3cbebec16..864103b707f4 100644
--- a/fs/notify/group.c
+++ b/fs/notify/group.c
@@ -124,6 +124,7 @@ 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 d3fea0bd89e2..d3005d95d530 100644
--- a/fs/notify/mark.c
+++ b/fs/notify/mark.c
@@ -105,6 +105,16 @@ 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);
+}
+
void fsnotify_put_mark(struct fsnotify_mark *mark)
{
if (atomic_dec_and_test(&mark->refcnt)) {
@@ -125,6 +135,72 @@ u32 fsnotify_recalc_mask(struct hlist_head *head)
return new_mask;
}
+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);
+}
+
/*
* Remove mark from inode / vfsmount list, group list, drop inode reference
* if we got one.
@@ -161,7 +237,6 @@ void fsnotify_detach_mark(struct fsnotify_mark *mark)
* __fsnotify_parent() lazily when next event happens on one of our
* children.
*/
-
list_del_init(&mark->g_list);
spin_unlock(&mark->lock);
@@ -508,6 +583,12 @@ 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));
}
void fsnotify_duplicate_mark(struct fsnotify_mark *new, struct fsnotify_mark *old)
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index 79467b239fcf..3de53a2b8944 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -79,6 +79,7 @@ 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
@@ -162,6 +163,8 @@ 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 {
@@ -350,6 +353,8 @@ 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,
--
2.19.1
