• Martin Lau's avatar
    ring-buffer: Fix polling on trace_pipe · 97b8ee84
    Martin Lau authored
    ring_buffer_poll_wait() should always put the poll_table to its wait_queue
    even there is immediate data available.  Otherwise, the following epoll and
    read sequence will eventually hang forever:
    
    1. Put some data to make the trace_pipe ring_buffer read ready first
    2. epoll_ctl(efd, EPOLL_CTL_ADD, trace_pipe_fd, ee)
    3. epoll_wait()
    4. read(trace_pipe_fd) till EAGAIN
    5. Add some more data to the trace_pipe ring_buffer
    6. epoll_wait() -> this epoll_wait() will block forever
    
    ~ During the epoll_ctl(efd, EPOLL_CTL_ADD,...) call in step 2,
      ring_buffer_poll_wait() returns immediately without adding poll_table,
      which has poll_table->_qproc pointing to ep_poll_callback(), to its
      wait_queue.
    ~ During the epoll_wait() call in step 3 and step 6,
      ring_buffer_poll_wait() cannot add ep_poll_callback() to its wait_queue
      because the poll_table->_qproc is NULL and it is how epoll works.
    ~ When there is new data available in step 6, ring_buffer does not know
      it has to call ep_poll_callback() because it is not in its wait queue.
      Hence, block forever.
    
    Other poll implementation seems to call poll_wait() unconditionally as the very
    first thing to do.  For example, tcp_poll() in tcp.c.
    
    Link: http://lkml.kernel.org/p/20140610060637.GA14045@devbig242.prn2.facebook.com
    
    Cc: stable@vger.kernel.org # 2.6.27
    Fixes: 2a2cc8f7 "ftrace: allow the event pipe to be polled"
    Reviewed-by: default avatarChris Mason <clm@fb.com>
    Signed-off-by: default avatarMartin Lau <kafai@fb.com>
    Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
    97b8ee84
ring_buffer.c 129 KB