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Subject[PATCH 2/4] epoll: introduce POLLFREE for ep_poll_callback()
Note: this patch is intentionally incomplete to simplify the review.
It ignores ep_unregister_pollwait() which plays with the same wqh.
See the next changes.

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.

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() should
be fine, poll/epoll doesn't use WQ_FLAG_EXCLUSIVE.

Reported-by: Maxime Bizon <mbizon@freebox.fr>
Cc: <stable@kernel.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
fs/eventpoll.c | 4 ++++
fs/signalfd.c | 5 +++++
include/asm-generic/poll.h | 2 ++
3 files changed, 11 insertions(+), 0 deletions(-)
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index aabdfc3..442bedb 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -844,6 +844,10 @@ 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)
+ list_del_init(&wait->task_list);
+
/*
* If the event mask does not contain any poll(2) event, we consider the
* descriptor to be disabled. This condition is likely the effect of the
diff --git a/fs/signalfd.c b/fs/signalfd.c
index 35d19ae..838ba21 100644
--- a/fs/signalfd.c
+++ b/fs/signalfd.c
@@ -34,6 +34,11 @@ void signalfd_cleanup(struct sighand_struct *sighand)
{
wait_queue_head_t *wqh = &sighand->signalfd_wqh;

+ if (likely(!waitqueue_active(wqh)))
+ return;
+
+ /* ask wait_queue_t->func() to remove_wait_queue() */
+ wake_up_poll(wqh, POLLHUP | POLLFREE);
BUG_ON(waitqueue_active(wqh));
}

diff --git a/include/asm-generic/poll.h b/include/asm-generic/poll.h
index 44bce83..9ce7f44 100644
--- a/include/asm-generic/poll.h
+++ b/include/asm-generic/poll.h
@@ -28,6 +28,8 @@
#define POLLRDHUP 0x2000
#endif

+#define POLLFREE 0x4000 /* currently only for epoll */
+
struct pollfd {
int fd;
short events;
--
1.5.5.1



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