tracing: Remove pointless memory barriers

Memory barriers are useful to ensure memory accesses from one CPU appear in
the original order as seen by other CPUs.

Some smp_rmb() and smp_wmb() are used, but they are not ordering multiple
memory accesses.

Remove them.

Cc: Masami Hiramatsu <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Gabriele Monaco <[email protected]>
Link: https://lore.kernel.org/[email protected]
Signed-off-by: Nam Cao <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: covanam

kernel/trace/rv/rv.c

@@ -675,8 +675,6 @@ static bool __read_mostly monitoring_on;
  */
 bool rv_monitoring_on(void)
 {
-	/* Ensures that concurrent monitors read consistent monitoring_on */
-	smp_rmb();
 	return READ_ONCE(monitoring_on);
 }
 
@@ -696,8 +694,6 @@ static ssize_t monitoring_on_read_data(struct file *filp, char __user *user_buf,
 static void turn_monitoring_off(void)
 {
 	WRITE_ONCE(monitoring_on, false);
-	/* Ensures that concurrent monitors read consistent monitoring_on */
-	smp_wmb();
 }
 
 static void reset_all_monitors(void)
@@ -713,8 +709,6 @@ static void reset_all_monitors(void)
 static void turn_monitoring_on(void)
 {
 	WRITE_ONCE(monitoring_on, true);
-	/* Ensures that concurrent monitors read consistent monitoring_on */
-	smp_wmb();
 }
 
 static void turn_monitoring_on_with_reset(void)

kernel/trace/trace.c

@@ -936,7 +936,6 @@ int tracing_is_enabled(void)
 	 * return the mirror variable of the state of the ring buffer.
 	 * It's a little racy, but we don't really care.
 	 */
-	smp_rmb();
 	return !global_trace.buffer_disabled;
 }
 
@@ -1107,8 +1106,6 @@ void tracer_tracing_on(struct trace_array *tr)
 	 * important to be fast than accurate.
 	 */
 	tr->buffer_disabled = 0;
-	/* Make the flag seen by readers */
-	smp_wmb();
 }
 
 /**
@@ -1640,8 +1637,6 @@ void tracer_tracing_off(struct trace_array *tr)
 	 * important to be fast than accurate.
 	 */
 	tr->buffer_disabled = 1;
-	/* Make the flag seen by readers */
-	smp_wmb();
 }
 
 /**
@@ -2710,8 +2705,6 @@ void trace_buffered_event_enable(void)
 
 static void enable_trace_buffered_event(void *data)
 {
-	/* Probably not needed, but do it anyway */
-	smp_rmb();
 	this_cpu_dec(trace_buffered_event_cnt);
 }