• Eric Wong's avatar
    epoll: prevent missed events on EPOLL_CTL_MOD · 128dd175
    Eric Wong authored
    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: default avatarEric 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: default avatar"Junchang(Jason) Wang" <junchang.wang@yale.edu>
    Cc: netdev@vger.kernel.org
    Cc: linux-fsdevel@vger.kernel.org
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    128dd175
eventpoll.c 54.7 KB