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authorOleg Nesterov <oleg@redhat.com>2012-02-24 20:07:11 +0100
committerLinus Torvalds <torvalds@linux-foundation.org>2012-02-24 11:42:50 -0800
commitd80e731ecab420ddcb79ee9d0ac427acbc187b4b (patch)
treee96a660b75b5bee8ae2c315878ec917b9c0da446 /fs/eventpoll.c
parent855a85f704026d5fe7de94fb1b765fe03404507f (diff)
downloadvexpress-lsk-d80e731ecab420ddcb79ee9d0ac427acbc187b4b.tar.gz
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 <mbizon@freebox.fr> Cc: <stable@kernel.org> Signed-off-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'fs/eventpoll.c')
-rw-r--r--fs/eventpoll.c4
1 files changed, 4 insertions, 0 deletions
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index aabdfc38cf2..34bbfc6dd8d 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);
/*