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    Subject[v2.6.34-stable 157/213] epoll: prevent missed events on EPOLL_CTL_MOD
    Date
    From: Eric Wong <normalperson@yhbt.net>

    -------------------
    This is a commit scheduled for the next v2.6.34 longterm release.
    http://git.kernel.org/?p=linux/kernel/git/paulg/longterm-queue-2.6.34.git
    If you see a problem with using this for longterm, please comment.
    -------------------

    commit 128dd1759d96ad36c379240f8b9463e8acfd37a1 upstream.

    EPOLL_CTL_MOD sets the interest mask before calling f_op->poll() to
    ensure events are not missed. Since the modifications to the interest
    mask are not protected by the same lock as ep_poll_callback, we need to
    ensure the change is visible to other CPUs calling ep_poll_callback.

    We also need to ensure f_op->poll() has an up-to-date view of past
    events which occured before we modified the interest mask. So this
    barrier also pairs with the barrier in wq_has_sleeper().

    This should guarantee either ep_poll_callback or f_op->poll() (or both)
    will notice the readiness of a recently-ready/modified item.

    This issue was encountered by Andreas Voellmy and Junchang(Jason) Wang in:
    http://thread.gmane.org/gmane.linux.kernel/1408782/

    Signed-off-by: Eric Wong <normalperson@yhbt.net>
    Cc: Hans Verkuil <hans.verkuil@cisco.com>
    Cc: Jiri Olsa <jolsa@redhat.com>
    Cc: Jonathan Corbet <corbet@lwn.net>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Cc: Davide Libenzi <davidel@xmailserver.org>
    Cc: Hans de Goede <hdegoede@redhat.com>
    Cc: Mauro Carvalho Chehab <mchehab@infradead.org>
    Cc: David Miller <davem@davemloft.net>
    Cc: Eric Dumazet <eric.dumazet@gmail.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Andreas Voellmy <andreas.voellmy@yale.edu>
    Tested-by: "Junchang(Jason) Wang" <junchang.wang@yale.edu>
    Cc: netdev@vger.kernel.org
    Cc: linux-fsdevel@vger.kernel.org
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
    ---
    fs/eventpoll.c | 22 +++++++++++++++++++++-
    1 file changed, 21 insertions(+), 1 deletion(-)

    diff --git a/fs/eventpoll.c b/fs/eventpoll.c
    index 34ca5ca9c3e8..f8a6c0876a7a 100644
    --- a/fs/eventpoll.c
    +++ b/fs/eventpoll.c
    @@ -1033,10 +1033,30 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even
    * otherwise we might miss an event that happens between the
    * f_op->poll() call and the new event set registering.
    */
    - epi->event.events = event->events;
    + epi->event.events = event->events; /* need barrier below */
    epi->event.data = event->data; /* protected by mtx */

    /*
    + * The following barrier has two effects:
    + *
    + * 1) Flush epi changes above to other CPUs. This ensures
    + * we do not miss events from ep_poll_callback if an
    + * event occurs immediately after we call f_op->poll().
    + * We need this because we did not take ep->lock while
    + * changing epi above (but ep_poll_callback does take
    + * ep->lock).
    + *
    + * 2) We also need to ensure we do not miss _past_ events
    + * when calling f_op->poll(). This barrier also
    + * pairs with the barrier in wq_has_sleeper (see
    + * comments for wq_has_sleeper).
    + *
    + * This barrier will now guarantee ep_poll_callback or f_op->poll
    + * (or both) will notice the readiness of an item.
    + */
    + smp_mb();
    +
    + /*
    * Get current event bits. We can safely use the file* here because
    * its usage count has been increased by the caller of this function.
    */
    --
    1.8.5.2


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