Message-ID: <CAJL_eksV6R2Je+qoBYAtvZ6pSb8d6HdNiBic7AH0TG3u2R0FzQ@mail.gmail.com>
Date:	Fri, 22 Jun 2012 18:27:16 -0700
From:	David Sharp <dhsharp@...gle.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, vnagarnaik@...gle.com
Subject: Re: [PATCH] ring-buffer: fix uninitialized read_stamp

On Fri, Jun 22, 2012 at 4:31 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Fri, 2012-06-22 at 13:50 -0700, David Sharp wrote:

>> On Thu, Jun 21, 2012 at 8:46 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
>> > OK, I did look at this more.
>>
>> >>  The only function that
>> >> sets read_stamp to a new valid value is rb_reset_reader_page(), which is called
>> >> only by rb_get_reader_page().
>> >
>> > Correct, which is the only way to get the reader page no matter how you
>> > read the buffer.
>> >
>> >>  rb_reset_reader_page() was not called when there
>> >> is data immediately available on the page to read (read < rb_page_size()). This
>> >> is the bug.
>> >
>> > It is not called?
>>
>> Correct. I was able to add WARN_ONs that showed that it reached "out:"
>> without reaching rb_reset_reader_page() first while still having a
>> poison value in cpu_buffer->read_stamp.
>
> NO! You didn't understand what I wrote. The writer can not get onto the
> reader page without a swap! The reader page is not part of the main ring
> buffer.

I understand that. I just thought the writer was getting onto the
reader page during a swap in the first read, the same read in which we
were getting invalid timestamps. But that doesn't make sense.

>> > Ah! I think this is where the bug you see happens. But your analysis is
>> > flawed.
>>
>> I admit I didn't look into it that closely, so flaws are quite
>> possible (also, I'm human).
>
> I didn't mean to offend, as this is not something that can be easily
> picked up.

None taken. I only meant I wasn't as rigorous as I could have been.

>> > If the ring buffer is currently empty, we swap an empty page for an
>> > empty page. But the writer ends up on the reader page preventing new
>> > swaps from taking place.
>> >
>> >    commit_page == reader_page
>> >
>> > If the writer starts writing, it will update the time stamp of the page.
>> > If your next read happens now, and it's just a single read, then you are
>> > correct that it will not update the read_stamp.
>> >
>> > I'm wondering if it would be better to just not do the swap, and return
>> > NULL when it is empty. This would also fix the problem, as the
>> > read_stamp will only be set when you actually have data.
>>
>> But we do have data... That's how we know we're getting invalid timestamps.
>
> I bet this never happens if you add a little data and then do a read. I
> only happens when you read from an empty ring buffer, then do a small
> write, and then read again. That's where it will get you.

Ah, see what you mean now. That seems possible.

>
>>
>> I don't quite understand what you're describing. Here's what I think
>> is happening though:
>>
>> When the ring buffer is reset, commit_page == tail_page == head_page.
>
> And at this moment, the writer is off the reader page and will not be on
> the reader page. The only way to get date at this point is if you do a
> swap.
>
>> rb_get_reader_page() will pick up the head page.
>
> Right. But this requires a read to happen. If you never do a read, then
> rb_get_reader_page will not pick up the head page. The problem is when
> you read from an empty ring buffer.
>
>
>>  Then a few (less than
>> 1 page worth) writes happen, on the tail_page which is currently (or
>> soon to be) also the reader_page. Now read == 0, but
>> rb_page_size(reader) is however many bytes have been written to the
>> page.
>
> Correct.
>
>>
>> So we have valid data on the reader page, but read_stamp has not been set yet.
>
> Yep
>
>>
>> >
>> > Or it may just be simpler to take your patch.
>>
>> Please? :)
>
> Now I think you may understand my patch.

Yeah, mostly. At least enough that I think it's worth testing. But Monday.

>> On Thu, Jun 21, 2012 at 8:56 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
>> > Does something like this work for you. Note, this is totally untested!
>> >
>> > -- Steve
>> >
>> > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
>> > index ad0239b..5943044 100644
>> > --- a/kernel/trace/ring_buffer.c
>> > +++ b/kernel/trace/ring_buffer.c
>> > @@ -3246,6 +3246,10 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
>> >        if (cpu_buffer->commit_page == cpu_buffer->reader_page)
>> >                goto out;
>> >
>> > +       /* Don't bother swapping if the ring buffer is empty */
>>
>> This doesn't make sense, since the ring buffer isn't empty in this
>> scenario. (I didn't bother testing it.)
>
> Yes it is! If you never did a read on the empty buffer, the *only* way
> to get what was written is to do the swap.
>
> The problem you are seeing is that you did a read on an empty buffer
> before writing. In this case the swap happens but all we have is an
> empty page with the write pointer (commit and tail) on it. When a write
> happens, and you do another read, *then* we do not do the swap and you
> get the stale time stamp.
>
> Thus, if you never let the first swap happen, you wont get into this
> scenario that you read and do not update the read_stamp.
>
>>
>> > +       if (rb_num_of_entries(cpu_buffer) == 0)
>>
>> Did you mean rb_page_entries(something?)
>
> Nope, I meant the actual ring buffer. If the ring buffer is empty, do
> not perform the swap. Because we only update the read stamp when we
> perform a swap, and it takes a write to happen before the page's
> timestamp is updated. If we do the swap before any writes happen, we get
> an invalid timestamp from the page.

Oops, I only asked because I'm blind, or can't type, or something,
because I couldn't find rb_num_of_entries().

>
> Now do you understand?
>
> -- Steve
>
>>
>> > +               goto out;
>> > +
>> >        /*
>> >         * Reset the reader page to size zero.
>> >         */
>> >
>> >
>
>
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