• Oleg Nesterov's avatar
    epoll: introduce POLLFREE to flush ->signalfd_wqh before kfree() · d80e731e
    Oleg Nesterov authored
    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
    __cleanup_sighand() is changed to do wake_up_poll(POLLFREE) if
    ->signalfd_wqh is not empty, we add the new signalfd_cleanup()
    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
    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.
    	- 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: default avatarMaxime Bizon <mbizon@freebox.fr>
    Cc: <stable@kernel.org>
    Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
signalfd.h 1.62 KB