Re: Potential Issue in Tracing Ring Buffer

[William Mcvicker]

On Mon, Nov 30, 2020 at 09:48:46AM -0500, Steven Rostedt wrote:
> On Thu, 26 Nov 2020 13:26:13 -0500
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> 
> > On Thu, 26 Nov 2020 06:52:45 +0100
> > Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > 
> > > On Tue, Nov 24, 2020 at 10:39:17PM +0000, J. Avila wrote:  
> > > > Hello,
> > > > 
> > > > In the ftrace logs we've collected internally, we have found that there are
> > > > situations where time seems to go backwards; this breaks userspace tools which
> > > > expect time to always go forward in these logs. For example, in this snippet
> > > > from a db845c running a 5.10-rc4 kernel[1] (thanks for getting us the trace,
> > > > John!), we see:    
> > > 
> > > Does the patch at:
> > > 	https://lore.kernel.org/r/20201125225654.1618966-1-minchan@xxxxxxxxxx
> > > 
> > > resolve this issue for you?
> > >   
> > 
> > I think I found the bug. Can you apply this patch and let me know if it
> > fixes the issue for you?
> > 
> > -- Steve
> > 
> > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > index dc83b3fa9fe7..bccaf88d3706 100644
> > --- a/kernel/trace/ring_buffer.c
> > +++ b/kernel/trace/ring_buffer.c
> > @@ -3291,7 +3291,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
> >  			/* Nothing came after this event between C and E */
> >  			info->delta = ts - info->after;
> >  			(void)rb_time_cmpxchg(&cpu_buffer->write_stamp,
> > -					      info->after, info->ts);
> > +					      info->after, ts);
> >  			info->ts = ts;
> >  		} else {
> >  			/*
> > 
> 
> Can I get a Tested-by from someone on the Google team, so that I can send
> this upstream? It already passed all my internal testing, but I want to
> make sure this is the fix for the issue I reference in the change log.
> 
> -- Steve
> 
> -- 
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> 
Hi Steve,

Thanks for the quick turnaround! Daniel and I have both tested your patch and
verified it's working on our end. Feel free to include:

Tested-by: Daniel Mentz <danielmentz@xxxxxxxxxx>
Tested-by: Will McVicker <willmcvicker@xxxxxxxxxx>

--Will