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

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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@vger.kernel.org
Acked-by: Christoph Lameter <cl@linux.com>
Reported-by: Uwe Kleine-Koenig <u.kleine-koenig@pengutronix.de>
Tested-by: Uwe Kleine-Koenig <u.kleine-koenig@pengutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
This commit is contained in:
Steven Rostedt
2015-03-17 10:40:38 -04:00
parent 06e5801b8c
commit 80a9b64e2c
1 changed files with 5 additions and 6 deletions
Download Patch File Download Diff File Expand all files Collapse all files

11
kernel/trace/ring_buffer.c
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@@ -2679,7 +2679,7 @@ static DEFINE_PER_CPU(unsigned int, current_context);
static __always_inline int trace_recursive_lock(void) 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; int bit;
if (in_interrupt()) { if (in_interrupt()) {
@@ -2696,18 +2696,17 @@ static __always_inline int trace_recursive_lock(void)
return 1; return 1;
val |= (1 << bit); val |= (1 << bit);
this_cpu_write(current_context, val); __this_cpu_write(current_context, val);
return 0; return 0;
} }
static __always_inline void trace_recursive_unlock(void) 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 &= val & (val - 1);
val &= this_cpu_read(current_context); __this_cpu_write(current_context, val);
this_cpu_write(current_context, val);
} }
#else #else