Commit 80a9b64e authored Mar 17, 2015 by Steven Rostedt

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@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>

parent 06e5801b

kernel/trace/ring_buffer.c

+5 −6

Original line number	Diff line number	Diff line
		@@ -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