From 0be53320ade39c226e9781c7ad72671625df6edf Mon Sep 17 00:00:00 2001 From: Martin Lau Date: Mon, 9 Jun 2014 23:06:42 -0700 Subject: [PATCH] ring-buffer: Fix polling on trace_pipe commit 97b8ee845393701edc06e27ccec2876ff9596019 upstream. 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 Fixes: 2a2cc8f7c4d0 "ftrace: allow the event pipe to be polled" Reviewed-by: Chris Mason Signed-off-by: Martin Lau Signed-off-by: Steven Rostedt [bwh: Backported to 3.2: the poll implementation looks rather different but does have a conditional return before and after the poll_wait() call; delete the return before it.] Signed-off-by: Ben Hutchings --- kernel/trace/trace.c | 2 -- 1 file changed, 2 deletions(-) diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c index c5a12a7d1287..0c348a63be49 100644 --- a/kernel/trace/trace.c +++ b/kernel/trace/trace.c @@ -3244,8 +3244,6 @@ tracing_poll_pipe(struct file *filp, poll_table *poll_table) */ return POLLIN | POLLRDNORM; } else { - if (!trace_empty(iter)) - return POLLIN | POLLRDNORM; poll_wait(filp, &trace_wait, poll_table); if (!trace_empty(iter)) return POLLIN | POLLRDNORM; -- 2.39.2