epoll: use RCU to protect wakeup_source in epitem
This prevents wakeup_source destruction when a user hits the item with
EPOLL_CTL_MOD while ep_poll_callback is running.
Tested with CONFIG_SPARSE_RCU_POINTER=y and "make fs/eventpoll.o C=2"
Signed-off-by: Eric Wong <normalperson@yhbt.net>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Arve Hjønnevåg <arve@android.com>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: NeilBrown <neilb@suse.de>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 0e5eda0..a3acf93 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -160,7 +160,7 @@
struct list_head fllink;
/* wakeup_source used when EPOLLWAKEUP is set */
- struct wakeup_source *ws;
+ struct wakeup_source __rcu *ws;
/* The structure that describe the interested events and the source fd */
struct epoll_event event;
@@ -538,6 +538,38 @@
}
}
+/* call only when ep->mtx is held */
+static inline struct wakeup_source *ep_wakeup_source(struct epitem *epi)
+{
+ return rcu_dereference_check(epi->ws, lockdep_is_held(&epi->ep->mtx));
+}
+
+/* call only when ep->mtx is held */
+static inline void ep_pm_stay_awake(struct epitem *epi)
+{
+ struct wakeup_source *ws = ep_wakeup_source(epi);
+
+ if (ws)
+ __pm_stay_awake(ws);
+}
+
+static inline bool ep_has_wakeup_source(struct epitem *epi)
+{
+ return rcu_access_pointer(epi->ws) ? true : false;
+}
+
+/* call when ep->mtx cannot be held (ep_poll_callback) */
+static inline void ep_pm_stay_awake_rcu(struct epitem *epi)
+{
+ struct wakeup_source *ws;
+
+ rcu_read_lock();
+ ws = rcu_dereference(epi->ws);
+ if (ws)
+ __pm_stay_awake(ws);
+ rcu_read_unlock();
+}
+
/**
* ep_scan_ready_list - Scans the ready list in a way that makes possible for
* the scan code, to call f_op->poll(). Also allows for
@@ -601,7 +633,7 @@
*/
if (!ep_is_linked(&epi->rdllink)) {
list_add_tail(&epi->rdllink, &ep->rdllist);
- __pm_stay_awake(epi->ws);
+ ep_pm_stay_awake(epi);
}
}
/*
@@ -670,7 +702,7 @@
list_del_init(&epi->rdllink);
spin_unlock_irqrestore(&ep->lock, flags);
- wakeup_source_unregister(epi->ws);
+ wakeup_source_unregister(ep_wakeup_source(epi));
/* At this point it is safe to free the eventpoll item */
kmem_cache_free(epi_cache, epi);
@@ -754,7 +786,7 @@
* callback, but it's not actually ready, as far as
* caller requested events goes. We can remove it here.
*/
- __pm_relax(epi->ws);
+ __pm_relax(ep_wakeup_source(epi));
list_del_init(&epi->rdllink);
}
}
@@ -986,7 +1018,7 @@
/* If this file is already in the ready list we exit soon */
if (!ep_is_linked(&epi->rdllink)) {
list_add_tail(&epi->rdllink, &ep->rdllist);
- __pm_stay_awake(epi->ws);
+ ep_pm_stay_awake_rcu(epi);
}
/*
@@ -1148,6 +1180,7 @@
static int ep_create_wakeup_source(struct epitem *epi)
{
const char *name;
+ struct wakeup_source *ws;
if (!epi->ep->ws) {
epi->ep->ws = wakeup_source_register("eventpoll");
@@ -1156,17 +1189,29 @@
}
name = epi->ffd.file->f_path.dentry->d_name.name;
- epi->ws = wakeup_source_register(name);
- if (!epi->ws)
+ ws = wakeup_source_register(name);
+
+ if (!ws)
return -ENOMEM;
+ rcu_assign_pointer(epi->ws, ws);
return 0;
}
-static void ep_destroy_wakeup_source(struct epitem *epi)
+/* rare code path, only used when EPOLL_CTL_MOD removes a wakeup source */
+static noinline void ep_destroy_wakeup_source(struct epitem *epi)
{
- wakeup_source_unregister(epi->ws);
- epi->ws = NULL;
+ struct wakeup_source *ws = ep_wakeup_source(epi);
+
+ rcu_assign_pointer(epi->ws, NULL);
+
+ /*
+ * wait for ep_pm_stay_awake_rcu to finish, synchronize_rcu is
+ * used internally by wakeup_source_remove, too (called by
+ * wakeup_source_unregister), so we cannot use call_rcu
+ */
+ synchronize_rcu();
+ wakeup_source_unregister(ws);
}
/*
@@ -1201,7 +1246,7 @@
if (error)
goto error_create_wakeup_source;
} else {
- epi->ws = NULL;
+ RCU_INIT_POINTER(epi->ws, NULL);
}
/* Initialize the poll table using the queue callback */
@@ -1249,7 +1294,7 @@
/* If the file is already "ready" we drop it inside the ready list */
if ((revents & event->events) && !ep_is_linked(&epi->rdllink)) {
list_add_tail(&epi->rdllink, &ep->rdllist);
- __pm_stay_awake(epi->ws);
+ ep_pm_stay_awake(epi);
/* Notify waiting tasks that events are available */
if (waitqueue_active(&ep->wq))
@@ -1290,7 +1335,7 @@
list_del_init(&epi->rdllink);
spin_unlock_irqrestore(&ep->lock, flags);
- wakeup_source_unregister(epi->ws);
+ wakeup_source_unregister(ep_wakeup_source(epi));
error_create_wakeup_source:
kmem_cache_free(epi_cache, epi);
@@ -1319,9 +1364,9 @@
pt._key = event->events;
epi->event.data = event->data; /* protected by mtx */
if (epi->event.events & EPOLLWAKEUP) {
- if (!epi->ws)
+ if (!ep_has_wakeup_source(epi))
ep_create_wakeup_source(epi);
- } else if (epi->ws) {
+ } else if (ep_has_wakeup_source(epi)) {
ep_destroy_wakeup_source(epi);
}
@@ -1359,7 +1404,7 @@
spin_lock_irq(&ep->lock);
if (!ep_is_linked(&epi->rdllink)) {
list_add_tail(&epi->rdllink, &ep->rdllist);
- __pm_stay_awake(epi->ws);
+ ep_pm_stay_awake(epi);
/* Notify waiting tasks that events are available */
if (waitqueue_active(&ep->wq))
@@ -1385,6 +1430,7 @@
unsigned int revents;
struct epitem *epi;
struct epoll_event __user *uevent;
+ struct wakeup_source *ws;
poll_table pt;
init_poll_funcptr(&pt, NULL);
@@ -1407,9 +1453,13 @@
* instead, but then epi->ws would temporarily be out of sync
* with ep_is_linked().
*/
- if (epi->ws && epi->ws->active)
- __pm_stay_awake(ep->ws);
- __pm_relax(epi->ws);
+ ws = ep_wakeup_source(epi);
+ if (ws) {
+ if (ws->active)
+ __pm_stay_awake(ep->ws);
+ __pm_relax(ws);
+ }
+
list_del_init(&epi->rdllink);
pt._key = epi->event.events;
@@ -1426,7 +1476,7 @@
if (__put_user(revents, &uevent->events) ||
__put_user(epi->event.data, &uevent->data)) {
list_add(&epi->rdllink, head);
- __pm_stay_awake(epi->ws);
+ ep_pm_stay_awake(epi);
return eventcnt ? eventcnt : -EFAULT;
}
eventcnt++;
@@ -1446,7 +1496,7 @@
* poll callback will queue them in ep->ovflist.
*/
list_add_tail(&epi->rdllink, &ep->rdllist);
- __pm_stay_awake(epi->ws);
+ ep_pm_stay_awake(epi);
}
}
}