tracing: Use flag buffer_disabled for irqsoff tracer

If the ring buffer is disabled and the irqsoff tracer records a trace it
will clear out its buffer and lose the data it had previously recorded.

Currently there's a callback when writing to the tracing_of file, but if
tracing is disabled via the function tracer trigger, it will not inform
the irqsoff tracer to stop recording.

By using the "mirror" flag (buffer_disabled) in the trace_array, that keeps
track of the status of the trace_array's buffer, it gives the irqsoff
tracer a fast way to know if it should record a new trace or not.
The flag may be a little behind the real state of the buffer, but it
should not affect the trace too much. It's more important for the irqsoff
tracer to be fast.

Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index c4c9296..0dc5071 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -226,9 +226,24 @@ cycle_t ftrace_now(int cpu)
        return ts;
 }
 
+/**
+ * tracing_is_enabled - Show if global_trace has been disabled
+ *
+ * Shows if the global trace has been enabled or not. It uses the
+ * mirror flag "buffer_disabled" to be used in fast paths such as for
+ * the irqsoff tracer. But it may be inaccurate due to races. If you
+ * need to know the accurate state, use tracing_is_on() which is a little
+ * slower, but accurate.
+ */
 int tracing_is_enabled(void)
 {
-       return tracing_is_on();
+       /*
+        * For quick access (irqsoff uses this in fast path), just
+        * 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;
 }
 
 /*
@@ -341,6 +356,23 @@ unsigned long trace_flags = TRACE_ITER_PRINT_PARENT | TRACE_ITER_PRINTK |
        TRACE_ITER_GRAPH_TIME | TRACE_ITER_RECORD_CMD | TRACE_ITER_OVERWRITE |
        TRACE_ITER_IRQ_INFO | TRACE_ITER_MARKERS | TRACE_ITER_FUNCTION;
 
+void tracer_tracing_on(struct trace_array *tr)
+{
+       if (tr->trace_buffer.buffer)
+               ring_buffer_record_on(tr->trace_buffer.buffer);
+       /*
+        * This flag is looked at when buffers haven't been allocated
+        * yet, or by some tracers (like irqsoff), that just want to
+        * know if the ring buffer has been disabled, but it can handle
+        * races of where it gets disabled but we still do a record.
+        * As the check is in the fast path of the tracers, it is more
+        * important to be fast than accurate.
+        */
+       tr->buffer_disabled = 0;
+       /* Make the flag seen by readers */
+       smp_wmb();
+}
+
 /**
  * tracing_on - enable tracing buffers
  *
@@ -349,15 +381,7 @@ unsigned long trace_flags = TRACE_ITER_PRINT_PARENT | TRACE_ITER_PRINTK |
  */
 void tracing_on(void)
 {
-       if (global_trace.trace_buffer.buffer)
-               ring_buffer_record_on(global_trace.trace_buffer.buffer);
-       /*
-        * This flag is only looked at when buffers haven't been
-        * allocated yet. We don't really care about the race
-        * between setting this flag and actually turning
-        * on the buffer.
-        */
-       global_trace.buffer_disabled = 0;
+       tracer_tracing_on(&global_trace);
 }
 EXPORT_SYMBOL_GPL(tracing_on);
 
@@ -551,6 +575,23 @@ void tracing_snapshot_alloc(void)
 EXPORT_SYMBOL_GPL(tracing_snapshot_alloc);
 #endif /* CONFIG_TRACER_SNAPSHOT */
 
+void tracer_tracing_off(struct trace_array *tr)
+{
+       if (tr->trace_buffer.buffer)
+               ring_buffer_record_off(tr->trace_buffer.buffer);
+       /*
+        * This flag is looked at when buffers haven't been allocated
+        * yet, or by some tracers (like irqsoff), that just want to
+        * know if the ring buffer has been disabled, but it can handle
+        * races of where it gets disabled but we still do a record.
+        * As the check is in the fast path of the tracers, it is more
+        * important to be fast than accurate.
+        */
+       tr->buffer_disabled = 1;
+       /* Make the flag seen by readers */
+       smp_wmb();
+}
+
 /**
  * tracing_off - turn off tracing buffers
  *
@@ -561,15 +602,7 @@ EXPORT_SYMBOL_GPL(tracing_snapshot_alloc);
  */
 void tracing_off(void)
 {
-       if (global_trace.trace_buffer.buffer)
-               ring_buffer_record_off(global_trace.trace_buffer.buffer);
-       /*
-        * This flag is only looked at when buffers haven't been
-        * allocated yet. We don't really care about the race
-        * between setting this flag and actually turning
-        * on the buffer.
-        */
-       global_trace.buffer_disabled = 1;
+       tracer_tracing_off(&global_trace);
 }
 EXPORT_SYMBOL_GPL(tracing_off);
 
@@ -580,13 +613,24 @@ void disable_trace_on_warning(void)
 }
 
 /**
+ * tracer_tracing_is_on - show real state of ring buffer enabled
+ * @tr : the trace array to know if ring buffer is enabled
+ *
+ * Shows real state of the ring buffer if it is enabled or not.
+ */
+int tracer_tracing_is_on(struct trace_array *tr)
+{
+       if (tr->trace_buffer.buffer)
+               return ring_buffer_record_is_on(tr->trace_buffer.buffer);
+       return !tr->buffer_disabled;
+}
+
+/**
  * tracing_is_on - show state of ring buffers enabled
  */
 int tracing_is_on(void)
 {
-       if (global_trace.trace_buffer.buffer)
-               return ring_buffer_record_is_on(global_trace.trace_buffer.buffer);
-       return !global_trace.buffer_disabled;
+       return tracer_tracing_is_on(&global_trace);
 }
 EXPORT_SYMBOL_GPL(tracing_is_on);
 
@@ -3958,7 +4002,7 @@ static int tracing_wait_pipe(struct file *filp)
                 *
                 * iter->pos will be 0 if we haven't read anything.
                 */
-               if (!tracing_is_enabled() && iter->pos)
+               if (!tracing_is_on() && iter->pos)
                        break;
        }
 
@@ -5631,15 +5675,10 @@ rb_simple_read(struct file *filp, char __user *ubuf,
               size_t cnt, loff_t *ppos)
 {
        struct trace_array *tr = filp->private_data;
-       struct ring_buffer *buffer = tr->trace_buffer.buffer;
        char buf[64];
        int r;
 
-       if (buffer)
-               r = ring_buffer_record_is_on(buffer);
-       else
-               r = 0;
-
+       r = tracer_tracing_is_on(tr);
        r = sprintf(buf, "%d\n", r);
 
        return simple_read_from_buffer(ubuf, cnt, ppos, buf, r);
@@ -5661,11 +5700,11 @@ rb_simple_write(struct file *filp, const char __user *ubuf,
        if (buffer) {
                mutex_lock(&trace_types_lock);
                if (val) {
-                       ring_buffer_record_on(buffer);
+                       tracer_tracing_on(tr);
                        if (tr->current_trace->start)
                                tr->current_trace->start(tr);
                } else {
-                       ring_buffer_record_off(buffer);
+                       tracer_tracing_off(tr);
                        if (tr->current_trace->stop)
                                tr->current_trace->stop(tr);
                }

diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
index b19d065..2aefbee 100644 (file)
--- a/kernel/trace/trace_irqsoff.c
+++ b/kernel/trace/trace_irqsoff.c
@@ -373,7 +373,7 @@ start_critical_timing(unsigned long ip, unsigned long parent_ip)
        struct trace_array_cpu *data;
        unsigned long flags;
 
-       if (likely(!tracer_enabled))
+       if (!tracer_enabled || !tracing_is_enabled())
                return;
 
        cpu = raw_smp_processor_id();
@@ -416,7 +416,7 @@ stop_critical_timing(unsigned long ip, unsigned long parent_ip)
        else
                return;
 
-       if (!tracer_enabled)
+       if (!tracer_enabled || !tracing_is_enabled())
                return;
 
        data = per_cpu_ptr(tr->trace_buffer.data, cpu);