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Subject[PATCH v2 1/2] 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>
---
fs/eventpoll.c | 4 ++++
fs/signalfd.c | 11 +++++++++++
include/asm-generic/poll.h | 2 ++
include/linux/signalfd.h | 5 ++++-
kernel/fork.c | 5 ++++-
5 files changed, 25 insertions(+), 2 deletions(-)
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);

/*
diff --git a/fs/signalfd.c b/fs/signalfd.c
index 492465b..79c1eea 100644
--- a/fs/signalfd.c
+++ b/fs/signalfd.c
@@ -30,6 +30,17 @@
#include <linux/signalfd.h>
#include <linux/syscalls.h>

+void signalfd_cleanup(struct sighand_struct *sighand)
+{
+ wait_queue_head_t *wqh = &sighand->signalfd_wqh;
+
+ if (likely(!waitqueue_active(wqh)))
+ return;
+
+ /* wait_queue_t->func(POLLFREE) should do remove_wait_queue() */
+ wake_up_poll(wqh, POLLHUP | POLLFREE);
+}
+
struct signalfd_ctx {
sigset_t sigmask;
};
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;
diff --git a/include/linux/signalfd.h b/include/linux/signalfd.h
index 3ff4961..247399b 100644
--- a/include/linux/signalfd.h
+++ b/include/linux/signalfd.h
@@ -61,13 +61,16 @@ static inline void signalfd_notify(struct task_struct *tsk, int sig)
wake_up(&tsk->sighand->signalfd_wqh);
}

+extern void signalfd_cleanup(struct sighand_struct *sighand);
+
#else /* CONFIG_SIGNALFD */

static inline void signalfd_notify(struct task_struct *tsk, int sig) { }

+static inline void signalfd_cleanup(struct sighand_struct *sighand) { }
+
#endif /* CONFIG_SIGNALFD */

#endif /* __KERNEL__ */

#endif /* _LINUX_SIGNALFD_H */
-
diff --git a/kernel/fork.c b/kernel/fork.c
index b77fd55..e2cd3e2 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -66,6 +66,7 @@
#include <linux/user-return-notifier.h>
#include <linux/oom.h>
#include <linux/khugepaged.h>
+#include <linux/signalfd.h>

#include <asm/pgtable.h>
#include <asm/pgalloc.h>
@@ -935,8 +936,10 @@ static int copy_sighand(unsigned long clone_flags, struct task_struct *tsk)

void __cleanup_sighand(struct sighand_struct *sighand)
{
- if (atomic_dec_and_test(&sighand->count))
+ if (atomic_dec_and_test(&sighand->count)) {
+ signalfd_cleanup(sighand);
kmem_cache_free(sighand_cachep, sighand);
+ }
}


--
1.5.5.1



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