On Mon, 23 Jul 2018 22:25:34 -0400 Steven Rostedt rostedt@goodmis.org wrote:

On Sat, 14 Jul 2018 01:28:15 +0900 Masami Hiramatsu mhiramat@kernel.org wrote:

...

Inherit the tracing on/off setting on ring_buffer to next trace buffer when taking a snapshot.

Taking a snapshot is done by swapping with backup ring buffer (max_tr_buffer). But since the tracing on/off setting is set in the ring buffer, when swapping it, 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 # 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 must be a bug, because user never know that each "ring_buffer" stores tracing-enable state and snapshot is done by swapping ring buffers.

This patch fixes above strange behavior.

OK, so the issue is that the tracing_on state is saved with the buffer itself. And when we swap the buffer, we also swap the state.

...

Fixes: commit debdd57f5145 ("tracing: Make a snapshot feature available from userspace") Signed-off-by: Masami Hiramatsu mhiramat@kernel.org Cc: Steven Rostedt rostedt@goodmis.org Cc: Ingo Molnar mingo@redhat.com Cc: Hiraku Toyooka hiraku.toyooka@cybertrust.co.jp Cc: stable@vger.kernel.org

---

include/linux/ring_buffer.h | 1 + kernel/trace/ring_buffer.c | 12 ++++++++++++ kernel/trace/trace.c | 6 ++++++ 3 files changed, 19 insertions(+)

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h index b72ebdff0b77..003d09ab308d 100644 --- a/include/linux/ring_buffer.h +++ b/include/linux/ring_buffer.h @@ -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); diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 6a46af21765c..4038ed74ab95 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -3227,6 +3227,18 @@ int ring_buffer_record_is_on(struct ring_buffer *buffer) } /**

• • 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.

I'd add a little more context here. Something like:

• 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.

Looks nice to me :) It makes the meaning clearer.

Thanks!

-- Steve

...

• */

+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.
• @cpu: The CPU buffer to stop

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c index 2556d8c097d2..bbd5a94a7ef1 100644 --- a/kernel/trace/trace.c +++ b/kernel/trace/trace.c @@ -1378,6 +1378,12 @@ 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);