Date:	Fri, 31 Jul 2009 15:13:39 +0200
From:	Robert Richter <robert.richter@....com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: WARNING: at kernel/trace/ring_buffer.c:2289
	rb_advance_reader+0x2e/0xc5()

Steve,

thanks for your detailed description. I found the root cause for the
warning, a patch is on the way. See also my comments below.

On 27.07.09 17:24:29, Steven Rostedt wrote:
> 
> Hi Robert,
> 
> 
> On Mon, 27 Jul 2009, Robert Richter wrote:
> 
> > Steven,
> > 
> > I have workloads for oprofile that trigger warnings like this:
> > 
> > ------------[ cut here ]------------
> > WARNING: at /dev/shm/.source/linux/kernel/trace/ring_buffer.c:2289 rb_advance_reader+0x2e/0xc5()
> 
> So this is this code:
> 
>         reader = rb_get_reader_page(cpu_buffer);
> 
>         /* This function should not be called when buffer is empty */
>         if (RB_WARN_ON(cpu_buffer, !reader))
>                 return;
> 
> 
> > Hardware name: Anaheim
> > Modules linked in:
> > Pid: 29, comm: events/2 Tainted: G        W  2.6.31-rc3-oprofile-x86_64-standard-00059-g5050dc2 #1
> > Call Trace:
> > [<ffffffff8106776f>] ? rb_advance_reader+0x2e/0xc5
> > [<ffffffff81039ffe>] warn_slowpath_common+0x77/0x8f
> > [<ffffffff8103a025>] warn_slowpath_null+0xf/0x11
> > [<ffffffff8106776f>] rb_advance_reader+0x2e/0xc5
> > [<ffffffff81068bda>] ring_buffer_consume+0xa0/0xd2
> > [<ffffffff81326933>] op_cpu_buffer_read_entry+0x21/0x9e
> > [<ffffffff810be3af>] ? __find_get_block+0x4b/0x165
> > [<ffffffff8132749b>] sync_buffer+0xa5/0x401
> > [<ffffffff810be3af>] ? __find_get_block+0x4b/0x165
> > [<ffffffff81326c1b>] ? wq_sync_buffer+0x0/0x78
> > [<ffffffff81326c76>] wq_sync_buffer+0x5b/0x78
> > [<ffffffff8104aa30>] worker_thread+0x113/0x1ac
> > [<ffffffff8104dd95>] ? autoremove_wake_function+0x0/0x38
> > [<ffffffff8104a91d>] ? worker_thread+0x0/0x1ac
> > [<ffffffff8104dc9a>] kthread+0x88/0x92
> > [<ffffffff8100bdba>] child_rip+0xa/0x20
> > [<ffffffff8104dc12>] ? kthread+0x0/0x92
> > [<ffffffff8100bdb0>] ? child_rip+0x0/0x20
> > ---[ end trace f561c0a58fcc89bd ]---
> > 
> > I am wondering when this may happen. It seems the following code in
> > rb_get_reader_page() triggers this:
> > 
> >         /* check if we caught up to the tail */
> >         reader = NULL;
> >         if (cpu_buffer->commit_page == cpu_buffer->reader_page)
> >                 goto out;
> 
> So you see that rb_advance_reader is called while the commit_page is the 
> same as the reader page? Which means that there's nothing left in the 
> buffer. This is interesting? Because in ring_buffer_consume we have:
> 
>         event = rb_buffer_peek(buffer, cpu, ts);
>         if (!event)
>                 goto out_unlock;
> 
>         rb_advance_reader(cpu_buffer);
> 
> Where rb_buffer_peak checks to see if there's anything in the buffer, if 
> there is, it returns an event, but does not increment the pointer. What 
> you see is that we got an event, but then the event went away? Is there 
> something else that could have read this buffer? You are not reading this 
> from an NMI or something?

There was only one thread reading from the buffer. Writes did go
always to the _other_ write buffer, and the buffer switch was only
made by the reading thread. So there is no one else using the buffers.

> 
> Or do you see any other WARN ONs happening? This would cause the locks to 
> not be taken for reads.

No, also no other warnings, also with lockdep enabled.

> 
> 
> > 
> > It is very hard to find out the conditions for equal commit and reader
> > pages.
> > 
> > Maybe you have any idea or could shortly explain the use of head,
> > tail, commit and reader page pointers? When an overrun happens?
> > (... or point me to some documentation.)
> 
> OK, I'll do it here, but there is documentation already posted, but I'm 
> too lazy to search for it (writing it is easier ;-)
> 
> The ring buffer is made up of 3 pointers. New writes go to the tail 
> pointer, and new reads, read from the head pointer.
> 
> To prevent races with the writer over writing the reader as the reader is 
> reading the buffer, the reader is given a separate page. When a read wants 
> to read the buffer, it swaps a blank page with the "head" page of the 
> buffer. If there is less then a page worth of data in the ring buffer, the 
> writer will also be on this page. When the write goes to the next page, it 
> simply moves back into the ring buffer because the next pointer of the 
> reader page points back into the ring buffer.
> 
> Since the write side is mosly lockless (and is totally lockless for the 32 
> queue), when a writer wants data it must first reserve space in the ring 
> buffer. Then the writer may copy directly into the ring buffer, and then 
> it "commits" it. This is where the commit pointer comes from. The 
> reserving of space from the ring buffer moves the tail. But the commit 
> pointer points to the last committed entry is. That is, data that the 
> reader can read. We can not allow a reader to read part of the ring buffer 
> that the writer has yet to write to.
> 
> When the writer is done and commits the changes, the commit pointer goes 
> to the end of that entry. This allows interrupts to preempt another 
> writer, but it will not overwrite the data of the preempted writer. A 
> write that interrupted another write will reserve data at the tail after 
> the data that was already reserved.
> 
> The head, tail and commit pointers are made up in two. There is a page and 
> index pointer. The page pointer points to the page and the index points to 
> the index in that page. A commit page pointer points to the page that 
> holds the commit and the commit index pointer points to the index in that 
> page.
> 
> To make things easier, if the writer wraps the ring buffer and comes back 
> to the head page, it will move the entire page.
> 
> The code you pointed out that shows the the reader == commit page, means 
> that there was less than a page worth of data when the reader swapped the 
> page from the buffer and the writer (or at least the commit page) was on 
> the page that was swapped. Thus the commit page is the same as the reader 
> page.
> 
> The conditions before hand checked to see if there was data left on the 
> page to read. If there was, then it would return the reader page, but when 
> the reader index == the commit index it means that there is no more data 
> left to read on that page and returns NULL.
> 
> This is strange because it is the same code that the peak uses, but the 
> previous call found something?

What I finally found out: rb_buffer_peek() was returning an event and
simultaneously advancing the reader. This is in case of padding events
and caused the 2nd rb_advance_reader() call to warn.

A patch is out now. I have a 2nd patch sent out too with some little
code rework.

Steve, thanks for your support.

-Robert

-- 
Advanced Micro Devices, Inc.
Operating System Research Center
email: robert.richter@....com

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