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SubjectRe: Potential Issue in Tracing Ring Buffer
On Thu, 26 Nov 2020 13:26:13 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Thu, 26 Nov 2020 06:52:45 +0100
> Greg KH <gregkh@linuxfoundation.org> 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@kernel.org
> >
> > 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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