On Tue, 27 May 2025 09:47:57 -0400
Steven Rostedt <rost...@goodmis.org> wrote:
> On Tue, 27 May 2025 12:54:44 +0900
> Masami Hiramatsu (Google) <mhira...@kernel.org> wrote:
>
> > Here is what I meant. As far as I ran my test, it looks good (it prevents
> > over-read by `cat per_cpu/cpu0/trace_pipe_raw`)
> >
> > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > index 5034bae02f08..de1831eb3446 100644
> > --- a/kernel/trace/ring_buffer.c
> > +++ b/kernel/trace/ring_buffer.c
> > @@ -5405,6 +5405,7 @@ rb_get_reader_page(struct ring_buffer_per_cpu
> > *cpu_buffer)
> > unsigned long flags;
> > int nr_loops = 0;
> > bool ret;
> > + u64 ts;
> >
> > local_irq_save(flags);
> > arch_spin_lock(&cpu_buffer->lock);
> > @@ -5423,6 +5424,18 @@ rb_get_reader_page(struct ring_buffer_per_cpu
> > *cpu_buffer)
> >
> > reader = cpu_buffer->reader_page;
> >
> > + /*
> > + * Now the page->commit is not cleared when it read.
> > + * Check whether timestamp is newer instead. We also don't
> > + * care the head_page is overwritten. In that case, timestamp
> > + * should be newer than reader timestamp too.
> > + */
> > + ts = cpu_buffer->head_page->page->time_stamp;
> > + if (ts < reader->page->time_stamp) {
>
> Hmm, I think this test may be too fragile. The head_page can be moved
> by the writer, and this would need to handle races.
Good point! Can we pick the page out from ring buffer as same
as reader_page? If its timestamp is newer, we push the reader
page (swap reader and head), or push back the header page.
>
> I found an issue with commit overflow and have a couple of bugs to fix that
> touches some of this code. Let's revisit after I get those fixed.
OK, let's review it.
BTW, we need a ring buffer test tool not depending on perf tool.
Thank you,
>
> Thanks,
>
> -- Steve
>
>
>
> > + reader = NULL;
> > + goto out;
> > + }
> > +
> > /* If there's more to read, return this page */
> > if (cpu_buffer->reader_page->read < rb_page_size(reader))
> > goto out;
> >
>
--
Masami Hiramatsu (Google) <mhira...@kernel.org>
