Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755198AbaG3Mbn (ORCPT ); Wed, 30 Jul 2014 08:31:43 -0400 Received: from ip4-83-240-18-248.cust.nbox.cz ([83.240.18.248]:36750 "EHLO ip4-83-240-18-248.cust.nbox.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752732AbaG3MPc (ORCPT ); Wed, 30 Jul 2014 08:15:32 -0400 From: Jiri Slaby To: stable@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Martin Lau , Steven Rostedt , Jiri Slaby Subject: [PATCH 3.12 60/94] ring-buffer: Fix polling on trace_pipe Date: Wed, 30 Jul 2014 14:14:49 +0200 Message-Id: <9b1829d6d4a1036a52bcdec52eab9b1d5b330369.1406722270.git.jslaby@suse.cz> X-Mailer: git-send-email 2.0.1 In-Reply-To: References: In-Reply-To: References: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Martin Lau 3.12-stable review patch. If anyone has any objections, please let me know. =============== 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 Signed-off-by: Jiri Slaby --- 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 15c4ae203885..a758ec217bc0 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 { -- 2.0.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/