diff options
Diffstat (limited to 'fs/eventpoll.c')
| -rw-r--r-- | fs/eventpoll.c | 37 |
1 files changed, 24 insertions, 13 deletions
diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 1e009cad8d5c..1b08556776ce 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -518,8 +518,13 @@ static void ep_remove_wait_queue(struct eppoll_entry *pwq) wait_queue_head_t *whead; rcu_read_lock(); - /* If it is cleared by POLLFREE, it should be rcu-safe */ - whead = rcu_dereference(pwq->whead); + /* + * If it is cleared by POLLFREE, it should be rcu-safe. + * If we read NULL we need a barrier paired with + * smp_store_release() in ep_poll_callback(), otherwise + * we rely on whead->lock. + */ + whead = smp_load_acquire(&pwq->whead); if (whead) remove_wait_queue(whead, &pwq->wait); rcu_read_unlock(); @@ -1003,17 +1008,6 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k struct epitem *epi = ep_item_from_wait(wait); struct eventpoll *ep = epi->ep; - if ((unsigned long)key & POLLFREE) { - ep_pwq_from_wait(wait)->whead = NULL; - /* - * whead = NULL above can race with ep_remove_wait_queue() - * which can do another remove_wait_queue() after us, so we - * can't use __remove_wait_queue(). whead->lock is held by - * the caller. - */ - list_del_init(&wait->task_list); - } - spin_lock_irqsave(&ep->lock, flags); /* @@ -1078,6 +1072,23 @@ out_unlock: if (pwake) ep_poll_safewake(&ep->poll_wait); + + if ((unsigned long)key & POLLFREE) { + /* + * If we race with ep_remove_wait_queue() it can miss + * ->whead = NULL and do another remove_wait_queue() after + * us, so we can't use __remove_wait_queue(). + */ + list_del_init(&wait->task_list); + /* + * ->whead != NULL protects us from the race with ep_free() + * or ep_remove(), ep_remove_wait_queue() takes whead->lock + * held by the caller. Once we nullify it, nothing protects + * ep/epi or even wait. + */ + smp_store_release(&ep_pwq_from_wait(wait)->whead, NULL); + } + return 1; } |