Re: [PATCH v13 2/6] ring-buffer: Introducing ring-buffer mapping functions

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 30 Jan 2024 16:22:06 +0000
Vincent Donnefort <vdonnefort@google.com> wrote:
> > > +static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
> > > +{
> > > +	struct trace_buffer_meta *meta = cpu_buffer->meta_page;
> > > +
> > > +	WRITE_ONCE(meta->reader.read, cpu_buffer->reader_page->read);
> > > +	WRITE_ONCE(meta->reader.id, cpu_buffer->reader_page->id);
> > > +	WRITE_ONCE(meta->reader.lost_events, cpu_buffer->lost_events);
> > > +
> > > +	WRITE_ONCE(meta->entries, local_read(&cpu_buffer->entries));
> > > +	WRITE_ONCE(meta->overrun, local_read(&cpu_buffer->overrun));
> > > +	WRITE_ONCE(meta->read, cpu_buffer->read);
> > > +}
> > > +
> > >  static void
> > >  rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
> > >  {
> > > @@ -5204,6 +5225,9 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
> > >  	cpu_buffer->lost_events = 0;
> > >  	cpu_buffer->last_overrun = 0;
> > >  
> > > +	if (cpu_buffer->mapped)  
> > 
> > There are some cpu_buffer->mapped are accessed via WRITE_ONCE/READ_ONCE()
> > but others are not. What makes those different?  
> 
> The cpu_buffer->mapped is READ_ONCE for the section where it is not protected
> with a lock. That is (in this version) only ring_buffer_swap_cpu().
> 
> That said... 
> 
> a. This is not enough protection at this level, Ideally the _map should also
> call synchronize_rcu() to make sure the _swap does see the ->mapped > 0.
> 
> b. With refcount for the snapshot in trace/trace.c, it is not possible for those
> functions to race. trace_arm_snapshot() and tracing_buffers_mmap() are mutually
> exclusive and already stabilized with the trace mutex.
> 
> So how about I completely remove those WRITE_ONCE/READ_ONCE and just rely on the
> protection given in trace/trace.c instead of duplicating it in
> trace/ring_buffer.c?

I would add a comment and replace the READ_ONCE with WARN_ON_ONCE():

	/* It is up to the callers to not swap mapped buffers */
	if (WARN_ON_ONCE(cpu_buffer_a->mapped || cpu_buffer_b->mapped)) {
		ret = -EBUSY;
		goto out;
	}


-- Steve