[lkml]   [2012]   [Feb]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
    Patch in this message
    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

    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.

    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 <>
    Cc: <>
    Signed-off-by: Oleg Nesterov <>
    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);

    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

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

     \ /
      Last update: 2012-02-22 18:43    [W:0.022 / U:0.536 seconds]
    ©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site