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 <c...@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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
