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fsnotify: Detach mark from object list when last reference is dropped
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Instead of removing mark from object list from fsnotify_detach_mark(),
remove the mark when last reference to the mark is dropped. This will
allow fanotify to wait for userspace response to event without having to
hold onto fsnotify_mark_srcu.

To avoid pinning inodes by elevated refcount (and thus e.g. delaying
file deletion) while someone holds mark reference, we detach connector
from the object also from fsnotify_destroy_marks() and not only after
removing last mark from the list as it was now.

Reviewed-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
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Jan Kara committed Apr 10, 2017
1 parent 1137514 commit 6b3f05d
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Showing 3 changed files with 105 additions and 77 deletions.
147 changes: 88 additions & 59 deletions fs/notify/mark.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -49,7 +49,13 @@
*
* A list of notification marks relating to inode / mnt is contained in
* fsnotify_mark_connector. That structure is alive as long as there are any
* marks in the list and is also protected by fsnotify_mark_srcu.
* marks in the list and is also protected by fsnotify_mark_srcu. A mark gets
* detached from fsnotify_mark_connector when last reference to the mark is
* dropped. Thus having mark reference is enough to protect mark->connector
* pointer and to make sure fsnotify_mark_connector cannot disappear. Also
* because we remove mark from g_list before dropping mark reference associated
* with that, any mark found through g_list is guaranteed to have
* mark->connector set until we drop group->mark_mutex.
*
* LIFETIME:
* Inode marks survive between when they are added to an inode and when their
Expand Down Expand Up @@ -103,26 +109,16 @@ void fsnotify_get_mark(struct fsnotify_mark *mark)
atomic_inc(&mark->refcnt);
}

void fsnotify_put_mark(struct fsnotify_mark *mark)
{
if (atomic_dec_and_test(&mark->refcnt)) {
spin_lock(&destroy_lock);
list_add(&mark->g_list, &destroy_list);
spin_unlock(&destroy_lock);
queue_delayed_work(system_unbound_wq, &reaper_work,
FSNOTIFY_REAPER_DELAY);
}
}

static void __fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
{
u32 new_mask = 0;
struct fsnotify_mark *mark;

assert_spin_locked(&conn->lock);
hlist_for_each_entry(mark, &conn->list, obj_list)
new_mask |= mark->mask;

hlist_for_each_entry(mark, &conn->list, obj_list) {
if (mark->flags & FSNOTIFY_MARK_FLAG_ATTACHED)
new_mask |= mark->mask;
}
if (conn->flags & FSNOTIFY_OBJ_TYPE_INODE)
conn->inode->i_fsnotify_mask = new_mask;
else if (conn->flags & FSNOTIFY_OBJ_TYPE_VFSMOUNT)
Expand All @@ -131,8 +127,9 @@ static void __fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)

/*
* Calculate mask of events for a list of marks. The caller must make sure
* connector cannot disappear under us (usually by holding a mark->lock or
* mark->group->mark_mutex for a mark on this list).
* connector and connector->inode cannot disappear under us. Callers achieve
* this by holding a mark->lock or mark->group->mark_mutex for a mark on this
* list.
*/
void fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
{
Expand Down Expand Up @@ -164,7 +161,6 @@ static void fsnotify_connector_destroy_workfn(struct work_struct *work)
}
}


static struct inode *fsnotify_detach_connector_from_object(
struct fsnotify_mark_connector *conn)
{
Expand All @@ -187,14 +183,34 @@ static struct inode *fsnotify_detach_connector_from_object(
return inode;
}

static struct inode *fsnotify_detach_from_object(struct fsnotify_mark *mark)
static void fsnotify_final_mark_destroy(struct fsnotify_mark *mark)
{
if (mark->group)
fsnotify_put_group(mark->group);
mark->free_mark(mark);
}

void fsnotify_put_mark(struct fsnotify_mark *mark)
{
struct fsnotify_mark_connector *conn;
struct inode *inode = NULL;
bool free_conn = false;

/* Catch marks that were actually never attached to object */
if (!mark->connector) {
if (atomic_dec_and_test(&mark->refcnt))
fsnotify_final_mark_destroy(mark);
return;
}

/*
* We have to be careful so that traversals of obj_list under lock can
* safely grab mark reference.
*/
if (!atomic_dec_and_lock(&mark->refcnt, &mark->connector->lock))
return;

conn = mark->connector;
spin_lock(&conn->lock);
hlist_del_init_rcu(&mark->obj_list);
if (hlist_empty(&conn->list)) {
inode = fsnotify_detach_connector_from_object(conn);
Expand All @@ -205,27 +221,40 @@ static struct inode *fsnotify_detach_from_object(struct fsnotify_mark *mark)
mark->connector = NULL;
spin_unlock(&conn->lock);

iput(inode);

if (free_conn) {
spin_lock(&destroy_lock);
conn->destroy_next = connector_destroy_list;
connector_destroy_list = conn;
spin_unlock(&destroy_lock);
queue_work(system_unbound_wq, &connector_reaper_work);
}

return inode;
/*
* Note that we didn't update flags telling whether inode cares about
* what's happening with children. We update these flags from
* __fsnotify_parent() lazily when next event happens on one of our
* children.
*/
spin_lock(&destroy_lock);
list_add(&mark->g_list, &destroy_list);
spin_unlock(&destroy_lock);
queue_delayed_work(system_unbound_wq, &reaper_work,
FSNOTIFY_REAPER_DELAY);
}

/*
* Remove mark from inode / vfsmount list, group list, drop inode reference
* if we got one.
* 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
* removed from group list before corresponding reference to it is dropped. In
* particular we rely on mark->connector being valid while we hold
* group->mark_mutex if we found the mark through g_list.
*
* Must be called with group->mark_mutex held. The caller must either hold
* reference to the mark or be protected by fsnotify_mark_srcu.
*/
void fsnotify_detach_mark(struct fsnotify_mark *mark)
{
struct inode *inode = NULL;
struct fsnotify_group *group = mark->group;

WARN_ON_ONCE(!mutex_is_locked(&group->mark_mutex));
Expand All @@ -234,31 +263,15 @@ void fsnotify_detach_mark(struct fsnotify_mark *mark)
!!(mark->flags & FSNOTIFY_MARK_FLAG_ATTACHED));

spin_lock(&mark->lock);

/* something else already called this function on this mark */
if (!(mark->flags & FSNOTIFY_MARK_FLAG_ATTACHED)) {
spin_unlock(&mark->lock);
return;
}

mark->flags &= ~FSNOTIFY_MARK_FLAG_ATTACHED;

inode = fsnotify_detach_from_object(mark);

/*
* Note that we didn't update flags telling whether inode cares about
* what's happening with children. We update these flags from
* __fsnotify_parent() lazily when next event happens on one of our
* children.
*/

list_del_init(&mark->g_list);

spin_unlock(&mark->lock);

if (inode)
iput(inode);

atomic_dec(&group->num_marks);

/* Drop mark reference acquired in fsnotify_add_mark_locked() */
Expand Down Expand Up @@ -458,7 +471,9 @@ static int fsnotify_add_mark_list(struct fsnotify_mark *mark,
hlist_for_each_entry(lmark, &conn->list, obj_list) {
last = lmark;

if ((lmark->group == mark->group) && !allow_dups) {
if ((lmark->group == mark->group) &&
(lmark->flags & FSNOTIFY_MARK_FLAG_ATTACHED) &&
!allow_dups) {
err = -EEXIST;
goto out_err;
}
Expand Down Expand Up @@ -509,7 +524,7 @@ int fsnotify_add_mark_locked(struct fsnotify_mark *mark,
mark->group = group;
list_add(&mark->g_list, &group->marks_list);
atomic_inc(&group->num_marks);
fsnotify_get_mark(mark); /* for i_list and g_list */
fsnotify_get_mark(mark); /* for g_list */
spin_unlock(&mark->lock);

ret = fsnotify_add_mark_list(mark, inode, mnt, allow_dups);
Expand Down Expand Up @@ -557,7 +572,8 @@ struct fsnotify_mark *fsnotify_find_mark(
return NULL;

hlist_for_each_entry(mark, &conn->list, obj_list) {
if (mark->group == group) {
if (mark->group == group &&
(mark->flags & FSNOTIFY_MARK_FLAG_ATTACHED)) {
fsnotify_get_mark(mark);
spin_unlock(&conn->lock);
return mark;
Expand Down Expand Up @@ -637,23 +653,38 @@ void fsnotify_detach_group_marks(struct fsnotify_group *group)
void fsnotify_destroy_marks(struct fsnotify_mark_connector __rcu **connp)
{
struct fsnotify_mark_connector *conn;
struct fsnotify_mark *mark;
struct fsnotify_mark *mark, *old_mark = NULL;
struct inode *inode;

while ((conn = fsnotify_grab_connector(connp))) {
/*
* We have to be careful since we can race with e.g.
* fsnotify_clear_marks_by_group() and once we drop the list
* lock, mark can get removed from the obj_list and destroyed.
* But we are holding mark reference so mark cannot be freed
* and calling fsnotify_destroy_mark() more than once is fine.
*/
mark = hlist_entry(conn->list.first, struct fsnotify_mark,
obj_list);
conn = fsnotify_grab_connector(connp);
if (!conn)
return;
/*
* We have to be careful since we can race with e.g.
* fsnotify_clear_marks_by_group() and once we drop the conn->lock, the
* list can get modified. However we are holding mark reference and
* thus our mark cannot be removed from obj_list so we can continue
* iteration after regaining conn->lock.
*/
hlist_for_each_entry(mark, &conn->list, obj_list) {
fsnotify_get_mark(mark);
spin_unlock(&conn->lock);
if (old_mark)
fsnotify_put_mark(old_mark);
old_mark = mark;
fsnotify_destroy_mark(mark, mark->group);
fsnotify_put_mark(mark);
spin_lock(&conn->lock);
}
/*
* Detach list from object now so that we don't pin inode until all
* mark references get dropped. It would lead to strange results such
* as delaying inode deletion or blocking unmount.
*/
inode = fsnotify_detach_connector_from_object(conn);
spin_unlock(&conn->lock);
if (old_mark)
fsnotify_put_mark(old_mark);
iput(inode);
}

/*
Expand Down Expand Up @@ -686,9 +717,7 @@ void fsnotify_mark_destroy_list(void)

list_for_each_entry_safe(mark, next, &private_destroy_list, g_list) {
list_del_init(&mark->g_list);
if (mark->group)
fsnotify_put_group(mark->group);
mark->free_mark(mark);
fsnotify_final_mark_destroy(mark);
}
}

Expand Down
4 changes: 2 additions & 2 deletions include/linux/fsnotify_backend.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -245,9 +245,9 @@ struct fsnotify_mark {
struct list_head g_list;
/* Protects inode / mnt pointers, flags, masks */
spinlock_t lock;
/* List of marks for inode / vfsmount [connector->lock] */
/* List of marks for inode / vfsmount [connector->lock, mark ref] */
struct hlist_node obj_list;
/* Head of list of marks for an object [mark->lock, group->mark_mutex] */
/* Head of list of marks for an object [mark ref] */
struct fsnotify_mark_connector *connector;
/* Events types to ignore [mark->lock, group->mark_mutex] */
__u32 ignored_mask;
Expand Down
31 changes: 15 additions & 16 deletions kernel/audit_tree.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -172,27 +172,18 @@ static unsigned long inode_to_key(const struct inode *inode)
/*
* Function to return search key in our hash from chunk. Key 0 is special and
* should never be present in the hash.
*
* Must be called with chunk->mark.lock held to protect from connector
* becoming NULL.
*/
static unsigned long __chunk_to_key(struct audit_chunk *chunk)
static unsigned long chunk_to_key(struct audit_chunk *chunk)
{
if (!chunk->mark.connector)
/*
* We have a reference to the mark so it should be attached to a
* connector.
*/
if (WARN_ON_ONCE(!chunk->mark.connector))
return 0;
return (unsigned long)chunk->mark.connector->inode;
}

static unsigned long chunk_to_key(struct audit_chunk *chunk)
{
unsigned long key;

spin_lock(&chunk->mark.lock);
key = __chunk_to_key(chunk);
spin_unlock(&chunk->mark.lock);
return key;
}

static inline struct list_head *chunk_hash(unsigned long key)
{
unsigned long n = key / L1_CACHE_BYTES;
Expand All @@ -202,7 +193,7 @@ static inline struct list_head *chunk_hash(unsigned long key)
/* hash_lock & entry->lock is held by caller */
static void insert_hash(struct audit_chunk *chunk)
{
unsigned long key = __chunk_to_key(chunk);
unsigned long key = chunk_to_key(chunk);
struct list_head *list;

if (!(chunk->mark.flags & FSNOTIFY_MARK_FLAG_ATTACHED))
Expand Down Expand Up @@ -263,6 +254,10 @@ static void untag_chunk(struct node *p)

mutex_lock(&entry->group->mark_mutex);
spin_lock(&entry->lock);
/*
* mark_mutex protects mark from getting detached and thus also from
* mark->connector->inode getting NULL.
*/
if (chunk->dead || !(entry->flags & FSNOTIFY_MARK_FLAG_ATTACHED)) {
spin_unlock(&entry->lock);
mutex_unlock(&entry->group->mark_mutex);
Expand Down Expand Up @@ -423,6 +418,10 @@ static int tag_chunk(struct inode *inode, struct audit_tree *tree)

mutex_lock(&old_entry->group->mark_mutex);
spin_lock(&old_entry->lock);
/*
* mark_mutex protects mark from getting detached and thus also from
* mark->connector->inode getting NULL.
*/
if (!(old_entry->flags & FSNOTIFY_MARK_FLAG_ATTACHED)) {
/* old_entry is being shot, lets just lie */
spin_unlock(&old_entry->lock);
Expand Down

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