[PATCH 3.14 20/92] ring-buffer: Replace this_cpu_*() with __this_cpu_*()

Greg Kroah-Hartman
2015-05-02T19:41:36Z

From: Steven Rostedt <rostedt@goodmis.org>

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@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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -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