• Josef Bacik's avatar
    trace: Fix epoll hang when we race with new entries · 4ce97dbf
    Josef Bacik authored
    Epoll on trace_pipe can sometimes hang in a weird case.  If the ring buffer is
    empty when we set waiters_pending but an event shows up exactly at that moment
    we can miss being woken up by the ring buffers irq work.  Since
    ring_buffer_empty() is inherently racey we will sometimes think that the buffer
    is not empty.  So we don't get woken up and we don't think there are any events
    even though there were some ready when we added the watch, which makes us hang.
    This patch fixes this by making sure that we are actually on the wait list
    before we set waiters_pending, and add a memory barrier to make sure
    ring_buffer_empty() is going to be correct.
    
    Link: http://lkml.kernel.org/p/1408989581-23727-1-git-send-email-jbacik@fb.com
    
    Cc: stable@vger.kernel.org # 3.10+
    Cc: Martin Lau <kafai@fb.com>
    Signed-off-by: default avatarJosef Bacik <jbacik@fb.com>
    Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
    4ce97dbf
ring_buffer.c 129 KB