| From | Greg Kroah-Hartman <> | Subject | [PATCH 3.14 20/92] ring-buffer: Replace this_cpu_*() with __this_cpu_*() | Date | Sat, 2 May 2015 21:02:35 +0200 |
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3.14-stable review patch. If anyone has any objections, please let me know.
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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
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