Messages in this thread Patch in this message | | | Date | Thu, 23 Feb 2012 16:44:38 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 4/4] epoll: ep_unregister_pollwait() can use the freed pwq->whead |
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On 02/22, Oleg Nesterov wrote: > > However this needs more locking. ep_remove_wait_queue() should take > ep->lock first to avoid the race and pin pwq->whead, then it needs > pwq->whead->lock for __remove_wait_queue(). > > This can obviously AB-BA deadlock with wake_up()->ep_poll_callback(), > so ep_remove_wait_queue() does the nasty lock + trylock-or-retry dance.
Or we can rely on the fact that sighand_cachep is SLAB_DESTROY_BY_RCU, and assume that ->whead is always rcu-protected if it can go away.
In this case we don't need 3/4 (although it makes sense to add the fat comment), and 4/4 can be simplified, see below.
ep_pwq_from_wait() is not really needed, it has a single caller. Just I tried to follow the coding style.
I'd prefer the explicit locking, but I don't really mind.
Oleg.
--- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -320,6 +320,11 @@ static inline int ep_is_linked(struct list_head *p) return !list_empty(p); } +static inline struct eppoll_entry *ep_pwq_from_wait(wait_queue_t *p) +{ + return container_of(p, struct eppoll_entry, wait); +} + /* Get the "struct epitem" from a wait queue pointer */ static inline struct epitem *ep_item_from_wait(wait_queue_t *p) { @@ -467,6 +472,17 @@ static void ep_poll_safewake(wait_queue_head_t *wq) put_cpu(); } +static void ep_remove_wait_queue(struct eppoll_entry *pwq) +{ + wait_queue_head_t *whead; + + rcu_read_lock(); + whead = rcu_dereference(pwq->whead); + if (whead) + remove_wait_queue(whead, &pwq->wait); + rcu_read_unlock(); +} + /* * This function unregisters poll callbacks from the associated file * descriptor. Must be called with "mtx" held (or "epmutex" if called from @@ -481,7 +497,7 @@ static void ep_unregister_pollwait(struct eventpoll *ep, struct epitem *epi) pwq = list_first_entry(lsthead, struct eppoll_entry, llink); list_del(&pwq->llink); - remove_wait_queue(pwq->whead, &pwq->wait); + ep_remove_wait_queue(pwq); kmem_cache_free(pwq_cache, pwq); } } @@ -845,8 +861,11 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k spin_lock_irqsave(&ep->lock, flags); /* the caller holds eppoll_entry->whead->lock */ - if ((unsigned long)key & POLLFREE) + if ((unsigned long)key & POLLFREE) { + /* can't use __remove_wait_queue(), we need list_del_init() */ list_del_init(&wait->task_list); + ep_pwq_from_wait(wait)->whead = NULL; + } /* * If the event mask does not contain any poll(2) event, we consider the
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