Re: [PATCH] ring-buffer: Fix polling on trace_pipe

[Chris Mason <clm@fb.com>]

On 06/10/2014 02:06 AM, Martin Lau wrote:
> 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.

Reviewed-by: Chris Mason <clm@fb.com>

This looked horribly wrong to me at first, but Martin walked me through
how the polling code is setting up waiters.  We have it in production here.