linux/include
Oleg Nesterov d80e731eca 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>
2012-02-24 11:42:50 -08:00
..
acpi Merge branches 'atomicio-apei', 'hotplug', 'sony-nvs-nosave' and 'thermal-netlink' into release 2012-01-23 19:47:06 -05:00
asm-generic epoll: introduce POLLFREE to flush ->signalfd_wqh before kfree() 2012-02-24 11:42:50 -08:00
crypto
drm drm: Fix authentication kernel crash 2012-01-25 09:27:45 +00:00
keys keys: add a "logon" key type 2012-01-17 22:39:40 -06:00
linux epoll: introduce POLLFREE to flush ->signalfd_wqh before kfree() 2012-02-24 11:42:50 -08:00
math-emu
media Merge branch 'v4l_for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mchehab/linux-media 2012-01-18 12:53:54 -08:00
misc
mtd mtd: document that MEMWRITE ioctl is NAND-specific 2012-01-09 18:18:36 +00:00
net netprio_cgroup: fix wrong memory access when NETPRIO_CGROUP=m 2012-02-10 15:08:57 -05:00
pcmcia
rdma rdma/core: Fix sparse warnings 2012-01-04 09:17:45 -08:00
rxrpc
scsi [SCSI] libfc: Declare local functions static 2012-01-16 12:45:48 +04:00
sound Merge branch 'fix/asoc' into for-linus 2012-01-31 15:13:14 +01:00
target target: Change target_submit_cmd() to return void 2012-02-07 06:41:04 +00:00
trace writeback: fix dereferencing NULL bdi->dev on trace_writeback_queue 2012-02-06 11:17:25 +08:00
video fbdev fixes for 3.3 2012-02-07 15:54:02 -08:00
xen Merge branch 'for-3.3/drivers' of git://git.kernel.dk/linux-block 2012-01-15 12:48:41 -08:00
Kbuild