From 1705f8074c75217f5810f7deb4518c39591b373c Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Sun, 28 Feb 2016 15:14:43 +0100
Subject: [PATCH 303/351] trace/writeback: Block cgroup path tracing on RT
X-NVConfidentiality: public

Yang reported that with CGROUP_WRITEBACK enabled the tracer triggers the
following backtrace:

BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:930
in_atomic(): 1, irqs_disabled(): 0, pid: 625, name: kworker/u16:3
INFO: lockdep is turned off.
Preemption disabled at:[<ffffffc000374a5c>] wb_writeback+0xec/0x830

CPU: 7 PID: 625 Comm: kworker/u16:3 Not tainted 4.4.1-rt5 #20
Hardware name: Freescale Layerscape 2085a RDB Board (DT)
Workqueue: writeback wb_workfn (flush-7:0)
Call trace:
[<ffffffc00008d708>] dump_backtrace+0x0/0x200
[<ffffffc00008d92c>] show_stack+0x24/0x30
[<ffffffc0007b0f40>] dump_stack+0x88/0xa8
[<ffffffc000127d74>] ___might_sleep+0x2ec/0x300
[<ffffffc000d5d550>] rt_spin_lock+0x38/0xb8
[<ffffffc0003e0548>] kernfs_path_len+0x30/0x90
[<ffffffc00036b360>] trace_event_raw_event_writeback_work_class+0xe8/0x2e8
[<ffffffc000374f90>] wb_writeback+0x620/0x830
[<ffffffc000376224>] wb_workfn+0x61c/0x950
[<ffffffc000110adc>] process_one_work+0x3ac/0xb30
[<ffffffc0001112fc>] worker_thread+0x9c/0x7a8
[<ffffffc00011a9e8>] kthread+0x190/0x1b0
[<ffffffc000086ca0>] ret_from_fork+0x10/0x30

Yang proposed to provide an unlocked function for accessing the cgroup path,
but that'd involve synchronize_sched() in a syscall, which is not desired
either. There was some discussion about using the inode number or the cgroup
id, but so far we have no working solution.

Disable the CGROUP_WRITEBACK path tracing for now when RT is enabled.

Reported-by: Yang Shi <yang.shi@linaro.org>
Link: http://lkml.kernel.org/r/1456528481-15936-1-git-send-email-yang.shi@linaro.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 include/trace/events/writeback.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index fff846b512e6..253ef2833c46 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -132,7 +132,7 @@ DEFINE_EVENT(writeback_dirty_inode_template, writeback_dirty_inode,
 );
 
 #ifdef CREATE_TRACE_POINTS
-#ifdef CONFIG_CGROUP_WRITEBACK
+#if defined(CONFIG_CGROUP_WRITEBACK) && !defined(CONFIG_PREEMPT_RT_FULL)
 
 static inline size_t __trace_wb_cgroup_size(struct bdi_writeback *wb)
 {
-- 
2.10.1