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Date:	Thu, 19 Mar 2015 18:02:16 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	stable@...r.kernel.org, Christoph Lameter <cl@...ux.com>,
	Uwe Kleine-Koenig <u.kleine-koenig@...gutronix.de>
Subject: [GIT PULL] ring-buffer: Replace this_cpu_*() with __this_cpu_*()


Linus,

The recursion code in the internals of the ftrace ring buffer requires
using "preempt_disable_notrace". But it has been discovered that
preempt_disable() is being used by this_cpu_read() in some architectures
and trace_cpu_read() is part of the recursion protection of the ring buffer.
The only reason this did not crash was due to the recursion protection
in other parts of ftrace. But if there's a path that does some kind
of function tracing without that protection, it will crash the kernel.

Use the __this_cpu_*() version instead which does not add preempt_disable()
or other unexpected functions to the per cpu code. Preemption is already
disabled at these paths, so the __this_cpu*() version should be used
anyawy.

Please pull the latest trace-fixes-v4.0-rc4 tree, which can be found at:


  git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git
trace-fixes-v4.0-rc4

Tag SHA1: 8c92b3f282f17acc4a17be91c7836e1442847270
Head SHA1: 9a22e2db723ae2c5eaf53efc40a1638620c1eb7a


Steven Rostedt (1):
      ring-buffer: Replace this_cpu_*() with __this_cpu_*()

----
 kernel/trace/ring_buffer.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)
---------------------------
commit 9a22e2db723ae2c5eaf53efc40a1638620c1eb7a
Author: Steven Rostedt <rostedt@...dmis.org>
Date:   Tue Mar 17 10:40:38 2015 -0400

    ring-buffer: Replace this_cpu_*() with __this_cpu_*()
    
    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
    
    Cc: stable@...r.kernel.org
    Acked-by: Christoph Lameter <cl@...ux.com>
    Reported-by: Uwe Kleine-KÃ=B6nig <u.kleine-koenig@...gutronix.de>
    Tested-by: Uwe Kleine-KÃ=B6nig <u.kleine-koenig@...gutronix.de>
    Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 5040d44fe5a3..922048a0f7ea 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2679,7 +2679,7 @@ 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()) {
@@ -2696,18 +2696,17 @@ 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
--
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