Patch "ring-buffer: Replace this_cpu_*() with __this_cpu_*()" has been added to the 3.14-stable tree

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

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

to the 3.14-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:
ring-buffer-replace-this_cpu_-with-__this_cpu_.patch
and it can be found in the queue-3.14 subdirectory.

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

From: Steven Rostedt <[email protected]>
Date: Tue, 17 Mar 2015 10:40:38 -0400
Subject: ring-buffer: Replace this_cpu_*() with __this_cpu_*()

From: Steven Rostedt <[email protected]>

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.

---
 kernel/trace/ring_buffer.c |   11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

@@ -2651,7 +2651,7 @@ static DEFINE_PER_CPU(unsigned int, curr
 
 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()) {

@@ -2668,18 +2668,17 @@ static __always_inline int trace_recursi
 		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

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

queue-3.14/ring-buffer-replace-this_cpu_-with-__this_cpu_.patch