[3.13.y-ckt stable] Patch "ring-buffer: Replace this_cpu_*() with __this_cpu_*()" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    ring-buffer: Replace this_cpu_*() with __this_cpu_*()

to the linux-3.13.y-queue branch of the 3.13.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11-ckt21.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 7fca83eac28cff5da18c21d665331ca1f684e227 Mon Sep 17 00:00:00 2001
From: Steven Rostedt <rostedt at goodmis.org>
Date: Tue, 17 Mar 2015 10:40:38 -0400
Subject: ring-buffer: Replace this_cpu_*() with __this_cpu_*()

commit 80a9b64e2c156b6523e7a01f2ba6e5d86e722814 upstream.

It has come to my attention that this_cpu_read/write are horrible on
architectures other than x86. Worse yet, they actually disable
preemption or interrupts! This caused some unexpected tracing results
on ARM.

   101.356868: preempt_count_add <-ring_buffer_lock_reserve
   101.356870: preempt_count_sub <-ring_buffer_lock_reserve

The ring_buffer_lock_reserve has recursion protection that requires
accessing a per cpu variable. But since preempt_disable() is traced, it
too got traced while accessing the variable that is suppose to prevent
recursion like this.

The generic version of this_cpu_read() and write() are:

 #define this_cpu_generic_read(pcp)					\
 ({	typeof(pcp) ret__;						\
	preempt_disable();						\
	ret__ = *this_cpu_ptr(&(pcp));					\
	preempt_enable();						\
	ret__;								\
 })

 #define this_cpu_generic_to_op(pcp, val, op)				\
 do {									\
	unsigned long flags;						\
	raw_local_irq_save(flags);					\
	*__this_cpu_ptr(&(pcp)) op val;					\
	raw_local_irq_restore(flags);					\
 } while (0)

Which is unacceptable for locations that know they are within preempt
disabled or interrupt disabled locations.

Paul McKenney stated that __this_cpu_() versions produce much better code on
other architectures than this_cpu_() does, if we know that the call is done in
a preempt disabled location.

I also changed the recursive_unlock() to use two local variables instead
of accessing the per_cpu variable twice.

Link: http://lkml.kernel.org/r/20150317114411.GE3589@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/20150317104038.312e73d1@gandalf.local.home

Acked-by: Christoph Lameter <cl at linux.com>
Reported-by: Uwe Kleine-Koenig <u.kleine-koenig at pengutronix.de>
Tested-by: Uwe Kleine-Koenig <u.kleine-koenig at pengutronix.de>
Signed-off-by: Steven Rostedt <rostedt at goodmis.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 kernel/trace/ring_buffer.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index a701f50..dc0a884 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2648,7 +2648,7 @@ static DEFINE_PER_CPU(unsigned int, current_context);

 static __always_inline int trace_recursive_lock(void)
 {
-	unsigned int val = this_cpu_read(current_context);
+	unsigned int val = __this_cpu_read(current_context);
 	int bit;

 	if (in_interrupt()) {
@@ -2665,18 +2665,17 @@ static __always_inline int trace_recursive_lock(void)
 		return 1;

 	val |= (1 << bit);
-	this_cpu_write(current_context, val);
+	__this_cpu_write(current_context, val);

 	return 0;
 }

 static __always_inline void trace_recursive_unlock(void)
 {
-	unsigned int val = this_cpu_read(current_context);
+	unsigned int val = __this_cpu_read(current_context);

-	val--;
-	val &= this_cpu_read(current_context);
-	this_cpu_write(current_context, val);
+	val &= val & (val - 1);
+	__this_cpu_write(current_context, val);
 }

 #else
--
1.9.1