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ring_buffer: tracing: Inherit the tracing setting to next ring buffer

PATCH In review Steven Rostedt 1 participant 1 message Jul 26, 2018

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Stable branch Tracing Tracing ring buffer
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Steven Rostedt

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PATCH 2/7 ring_buffer: tracing: Inherit the tracing setting to next ring buffer

From: Masami Hiramatsu <[email protected]>

Maintain the tracing on/off setting of the ring_buffer when switching to the trace buffer snapshot.

Taking a snapshot is done by swapping the backup ring buffer (max_tr_buffer). But since the tracing on/off setting is defined by the ring buffer, when swapping it, the tracing on/off setting can also be changed. This causes a strange result like below:

/sys/kernel/debug/tracing # cat tracing_on
1
/sys/kernel/debug/tracing # echo 0 > tracing_on
/sys/kernel/debug/tracing # cat tracing_on
0
/sys/kernel/debug/tracing # echo 1 > snapshot
/sys/kernel/debug/tracing # cat tracing_on
1
/sys/kernel/debug/tracing # echo 1 > snapshot
/sys/kernel/debug/tracing # cat tracing_on
0

We don't touch tracing_on, but snapshot changes tracing_on setting each time. This is an anomaly, because user doesn't know that each "ring_buffer" stores its own tracing-enable state and the snapshot is done by swapping ring buffers.

Link http://lkml.kernel.org/r/153149929558.11274.11730609978254724394.stgit@devbox open here
Cc Ingo Molnar <[email protected]>
Cc Shuah Khan <[email protected]>
Cc Tom Zanussi <[email protected]>
Cc Hiraku Toyooka <[email protected]>
Cc [email protected]
Fixes debdd57f5145 ("tracing: Make a snapshot feature available from userspace")
Signed-off-by Masami Hiramatsu <[email protected]>
[ Updated commit log and comment in the code ]
Signed-off-by Steven Rostedt (VMware) <[email protected]>
---
 include/linux/ring_buffer.h |  1 +
 kernel/trace/ring_buffer.c  | 16 ++++++++++++++++
 kernel/trace/trace.c        |  6 ++++++
 3 files changed, 23 insertions(+)
include/linux/ring_buffer.h +1 -0
index b72ebdff0b77..003d09ab308d 100644
@@ -165,6 +165,7 @@ void ring_buffer_record_enable(struct ring_buffer *buffer);
 void ring_buffer_record_off(struct ring_buffer *buffer);
 void ring_buffer_record_on(struct ring_buffer *buffer);
 int ring_buffer_record_is_on(struct ring_buffer *buffer);
+int ring_buffer_record_is_set_on(struct ring_buffer *buffer);
 void ring_buffer_record_disable_cpu(struct ring_buffer *buffer, int cpu);
 void ring_buffer_record_enable_cpu(struct ring_buffer *buffer, int cpu);
kernel/trace/ring_buffer.c +16 -0
index 6a46af21765c..0b0b688ea166 100644
@@ -3226,6 +3226,22 @@ int ring_buffer_record_is_on(struct ring_buffer *buffer)
 	return !atomic_read(&buffer->record_disabled);
 }
 
+/**
+ * ring_buffer_record_is_set_on - return true if the ring buffer is set writable
+ * @buffer: The ring buffer to see if write is set enabled
+ *
+ * Returns true if the ring buffer is set writable by ring_buffer_record_on().
+ * Note that this does NOT mean it is in a writable state.
+ *
+ * It may return true when the ring buffer has been disabled by
+ * ring_buffer_record_disable(), as that is a temporary disabling of
+ * the ring buffer.
+ */
+int ring_buffer_record_is_set_on(struct ring_buffer *buffer)
+{
+	return !(atomic_read(&buffer->record_disabled) & RB_BUFFER_OFF);
+}
+
 /**
  * ring_buffer_record_disable_cpu - stop all writes into the cpu_buffer
  * @buffer: The ring buffer to stop writes to.
kernel/trace/trace.c +6 -0
index 87cf25171fb8..823687997b01 100644
@@ -1373,6 +1373,12 @@ update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu)
 
 	arch_spin_lock(&tr->max_lock);
 
+	/* Inherit the recordable setting from trace_buffer */
+	if (ring_buffer_record_is_set_on(tr->trace_buffer.buffer))
+		ring_buffer_record_on(tr->max_buffer.buffer);
+	else
+		ring_buffer_record_off(tr->max_buffer.buffer);
+
 	swap(tr->trace_buffer.buffer, tr->max_buffer.buffer);
 
 	__update_max_tr(tr, tsk, cpu);
Signature · 1 line
2.17.1