Patch "printk: use rcuidle console tracepoint" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

printk: use rcuidle console tracepoint

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
printk-use-rcuidle-console-tracepoint.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Sergey Senozhatsky <[email protected]>
Date: Sat, 18 Feb 2017 03:42:54 -0800
Subject: printk: use rcuidle console tracepoint

From: Sergey Senozhatsky <[email protected]>

commit fc98c3c8c9dcafd67adcce69e6ce3191d5306c9c upstream.

Use rcuidle console tracepoint because, apparently, it may be issued from an idle CPU:

hw-breakpoint: Failed to enable monitor mode on CPU 0.
hw-breakpoint: CPU 0 failed to disable vector catch

===============================
[ ERR: suspicious RCU usage. ]
4.10.0-rc8-next-20170215+ #119 Not tainted
-------------------------------
./include/trace/events/printk.h:32 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

RCU used illegally from idle CPU!
rcu_scheduler_active = 2, debug_locks = 0
RCU used illegally from extended quiescent state!
2 locks held by swapper/0/0:
#0: (cpu_pm_notifier_lock){......}, at: [<c0237e2c>] cpu_pm_exit+0x10/0x54 #1: (console_lock){+.+.+.}, at: [<c01ab350>] vprintk_emit+0x264/0x474

  stack backtrace:
  CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.10.0-rc8-next-20170215+ #119
  Hardware name: Generic OMAP4 (Flattened Device Tree)
    console_unlock
    vprintk_emit
    vprintk_default
    printk
    reset_ctrl_regs
    dbg_cpu_pm_notify
    notifier_call_chain
    cpu_pm_exit
    omap_enter_idle_coupled
    cpuidle_enter_state
    cpuidle_enter_state_coupled
    do_idle
    cpu_startup_entry
    start_kernel

This RCU warning, however, is suppressed by lockdep_off() in printk(). lockdep_off() increments the ->lockdep_recursion counter and thus disables RCU_LOCKDEP_WARN() and debug_lockdep_rcu_enabled(), which want lockdep to be enabled "current->lockdep_recursion == 0".

---
 kernel/printk/printk.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -1436,7 +1436,7 @@ static void call_console_drivers(int lev
 {
 	struct console *con;
 
-	trace_console(text, len);
+	trace_console_rcuidle(text, len);
 
 	if (level >= console_loglevel && !ignore_loglevel)
 		return;

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/printk-use-rcuidle-console-tracepoint.patch