From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760860AbaGPD1F (ORCPT ); Tue, 15 Jul 2014 23:27:05 -0400 Received: from cdptpa-outbound-snat.email.rr.com ([107.14.166.226]:55746 "EHLO cdptpa-oedge-vip.email.rr.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1760775AbaGPD0j (ORCPT ); Tue, 15 Jul 2014 23:26:39 -0400 Message-Id: <20140716032637.509579203@goodmis.org> User-Agent: quilt/0.63-1 Date: Tue, 15 Jul 2014 23:24:59 -0400 From: Steven Rostedt To: linux-kernel@vger.kernel.org Cc: Linus Torvalds , Ingo Molnar , Andrew Morton , Chris Mason , Martin Lau Subject: [PATCH 4/4] ring-buffer: Fix polling on trace_pipe References: <20140716032455.761976900@goodmis.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline; filename=0004-ring-buffer-Fix-polling-on-trace_pipe.patch X-RR-Connecting-IP: 107.14.168.130:25 X-Cloudmark-Score: 0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Martin Lau 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: 2a2cc8f7c4d0 "ftrace: allow the event pipe to be polled" Reviewed-by: Chris Mason Signed-off-by: Martin Lau Signed-off-by: Steven Rostedt --- kernel/trace/ring_buffer.c | 4 ---- 1 file changed, 4 deletions(-) diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 7c56c3d06943..ff7027199a9a 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -616,10 +616,6 @@ int ring_buffer_poll_wait(struct ring_buffer *buffer, int cpu, struct ring_buffer_per_cpu *cpu_buffer; struct rb_irq_work *work; - if ((cpu == RING_BUFFER_ALL_CPUS && !ring_buffer_empty(buffer)) || - (cpu != RING_BUFFER_ALL_CPUS && !ring_buffer_empty_cpu(buffer, cpu))) - return POLLIN | POLLRDNORM; - if (cpu == RING_BUFFER_ALL_CPUS) work = &buffer->irq_work; else { -- 2.0.0