From d80e731ecab420ddcb79ee9d0ac427acbc187b4b Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Fri, 24 Feb 2012 20:07:11 +0100 Subject: epoll: introduce POLLFREE to flush ->signalfd_wqh before kfree() This patch is intentionally incomplete to simplify the review. It ignores ep_unregister_pollwait() which plays with the same wqh. See the next change. epoll assumes that the EPOLL_CTL_ADD'ed file controls everything f_op->poll() needs. In particular it assumes that the wait queue can't go away until eventpoll_release(). This is not true in case of signalfd, the task which does EPOLL_CTL_ADD uses its ->sighand which is not connected to the file. This patch adds the special event, POLLFREE, currently only for epoll. It expects that init_poll_funcptr()'ed hook should do the necessary cleanup. Perhaps it should be defined as EPOLLFREE in eventpoll. __cleanup_sighand() is changed to do wake_up_poll(POLLFREE) if ->signalfd_wqh is not empty, we add the new signalfd_cleanup() helper. ep_poll_callback(POLLFREE) simply does list_del_init(task_list). This make this poll entry inconsistent, but we don't care. If you share epoll fd which contains our sigfd with another process you should blame yourself. signalfd is "really special". I simply do not know how we can define the "right" semantics if it used with epoll. The main problem is, epoll calls signalfd_poll() once to establish the connection with the wait queue, after that signalfd_poll(NULL) returns the different/inconsistent results depending on who does EPOLL_CTL_MOD/signalfd_read/etc. IOW: apart from sigmask, signalfd has nothing to do with the file, it works with the current thread. In short: this patch is the hack which tries to fix the symptoms. It also assumes that nobody can take tasklist_lock under epoll locks, this seems to be true. Note: - we do not have wake_up_all_poll() but wake_up_poll() is fine, poll/epoll doesn't use WQ_FLAG_EXCLUSIVE. - signalfd_cleanup() uses POLLHUP along with POLLFREE, we need a couple of simple changes in eventpoll.c to make sure it can't be "lost". Reported-by: Maxime Bizon Cc: Signed-off-by: Oleg Nesterov Signed-off-by: Linus Torvalds --- fs/eventpoll.c | 4 ++++ 1 file changed, 4 insertions(+) (limited to 'fs/eventpoll.c') diff --git a/fs/eventpoll.c b/fs/eventpoll.c index aabdfc3..34bbfc6 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -842,6 +842,10 @@ 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; + /* the caller holds eppoll_entry->whead->lock */ + if ((unsigned long)key & POLLFREE) + list_del_init(&wait->task_list); + spin_lock_irqsave(&ep->lock, flags); /* -- cgit v1.1 From 971316f0503a5c50633d07b83b6db2f15a3a5b00 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Fri, 24 Feb 2012 20:07:29 +0100 Subject: epoll: ep_unregister_pollwait() can use the freed pwq->whead signalfd_cleanup() ensures that ->signalfd_wqh is not used, but this is not enough. eppoll_entry->whead still points to the memory we are going to free, ep_unregister_pollwait()->remove_wait_queue() is obviously unsafe. Change ep_poll_callback(POLLFREE) to set eppoll_entry->whead = NULL, change ep_unregister_pollwait() to check pwq->whead != NULL under rcu_read_lock() before remove_wait_queue(). We add the new helper, ep_remove_wait_queue(), for this. This works because sighand_cachep is SLAB_DESTROY_BY_RCU and because ->signalfd_wqh is initialized in sighand_ctor(), not in copy_sighand. ep_unregister_pollwait()->remove_wait_queue() can play with already freed and potentially reused ->sighand, but this is fine. This memory must have the valid ->signalfd_wqh until rcu_read_unlock(). Reported-by: Maxime Bizon Cc: Signed-off-by: Oleg Nesterov Signed-off-by: Linus Torvalds --- fs/eventpoll.c | 30 +++++++++++++++++++++++++++--- 1 file changed, 27 insertions(+), 3 deletions(-) (limited to 'fs/eventpoll.c') diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 34bbfc6..ea54cde 100644 --- 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 +472,18 @@ 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(); + /* If it is cleared by POLLFREE, it should be rcu-safe */ + 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 +498,7 @@ 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); } } @@ -842,9 +859,16 @@ 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; - /* the caller holds eppoll_entry->whead->lock */ - if ((unsigned long)key & POLLFREE) + 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); -- cgit v1.1