Re: [PATCH 2/3] [BUGFIX] ring_buffer: tracing: Inherit the tracing setting to next ring buffer

[Masami Hiramatsu <mhiramat@kernel.org>]

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);
> 


-- 
Masami Hiramatsu <mhiramat@kernel.org>