From eea1d585917c538d90bc26fda5d8e53796feada2 Mon Sep 17 00:00:00 2001 From: Eric Wong Date: Tue, 30 Apr 2013 15:27:39 -0700 Subject: [PATCH] epoll: use RCU to protect wakeup_source in epitem MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Cc: Alexander Viro Cc: Arve Hjønnevåg Cc: Davide Libenzi Cc: Eric Dumazet Cc: NeilBrown Cc: "Rafael J. Wysocki" Cc: "Paul E. McKenney" Cc: Oleg Nesterov Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- fs/eventpoll.c | 92 ++++++++++++++++++++++++++++++++++++++------------ 1 file changed, 71 insertions(+), 21 deletions(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 0e5eda068520..a3acf936c72a 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -160,7 +160,7 @@ struct epitem { 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 @@ static void ep_unregister_pollwait(struct eventpoll *ep, struct epitem *epi) } } +/* 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 @@ static int ep_scan_ready_list(struct eventpoll *ep, */ 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 @@ static int ep_remove(struct eventpoll *ep, struct epitem *epi) 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 @@ static int ep_read_events_proc(struct eventpoll *ep, struct list_head *head, * 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 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k /* 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 reverse_path_check(void) 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 @@ static int ep_create_wakeup_source(struct epitem *epi) } 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 @@ static int ep_insert(struct eventpoll *ep, struct epoll_event *event, 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 @@ static int ep_insert(struct eventpoll *ep, struct epoll_event *event, /* 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 @@ error_unregister: 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 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even 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 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even 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 @@ static int ep_send_events_proc(struct eventpoll *ep, struct list_head *head, 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 @@ static int ep_send_events_proc(struct eventpoll *ep, struct list_head *head, * 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 @@ static int ep_send_events_proc(struct eventpoll *ep, struct list_head *head, 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 @@ static int ep_send_events_proc(struct eventpoll *ep, struct list_head *head, * 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); } } }