Subject: Re: [PATCH 1/2] ftrace: disable tracing on resize

From: Pekka Paalanen <[email-masked]>
Date: Wed, 12 Nov 2008 21:56:30 +0200

On Mon, 10 Nov 2008 22:18:19 -0500 (EST)
Steven Rostedt <[email-masked]> wrote:

> 
> Pekka,
> 
> I forgot to add you to the Cc list. Could you test this patch to see if it 
> fixes the bug you reported. It fixed a bug I was able to reproduce, but I 
> was never able to reproduce the exact same bug you have seen.

Thanks, Steve.

I didn't have time to try it myself yet, but a kind soul tried it for me.
He used 2.6.28-rc4 and was immediately able to trigger the bug by just:
cd /debug/tracing
echo 0 > tracing_enabled
echo 200 > trace_entries

Then he applied the patch you referred to, and the bug is still present.
He has a 32-bit x86 amd system, and I'm on x86_64.

I don't know why you couldn't reproduce it, maybe you have all the tracers
enabled in the kernel config, when we only have MMIOTRACE.

I can't promise to look into this deeper in the near future :-/

> On Mon, 10 Nov 2008, Steven Rostedt wrote:
> 
> > Impact: fix for bug on resize
> > 
> > This patch addresses the bug found here:
> > 
> >  http://bugzilla.kernel.org/show_bug.cgi?id=11996
> > 
> > When ftrace converted to the new unified trace buffer, the resizing of
> > the buffer was not protected as much as it was originally. If tracing
> > is performed while the resize occurs, then the buffer can be corrupted.
> > 
> > This patch disables all ftrace buffer modifications before a resize
> > takes place.
> > 
> > Signed-off-by: Steven Rostedt <[email-masked]>
> > ---
> >  kernel/trace/trace.c |   17 ++++++++++++++++-
> >  1 files changed, 16 insertions(+), 1 deletions(-)
> > 
> > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > index 9f3b478..abfa810 100644
> > --- a/kernel/trace/trace.c
> > +++ b/kernel/trace/trace.c
> > @@ -2676,7 +2676,7 @@ tracing_entries_write(struct file *filp, const char __user *ubuf,
> >  {
> >  	unsigned long val;
> >  	char buf[64];
> > -	int ret;
> > +	int ret, cpu;
> >  	struct trace_array *tr = filp->private_data;
> >  
> >  	if (cnt >= sizeof(buf))
> > @@ -2704,6 +2704,14 @@ tracing_entries_write(struct file *filp, const char __user *ubuf,
> >  		goto out;
> >  	}
> >  
> > +	/* disable all cpu buffers */
> > +	for_each_tracing_cpu(cpu) {
> > +		if (global_trace.data[cpu])
> > +			atomic_inc(&global_trace.data[cpu]->disabled);
> > +		if (max_tr.data[cpu])
> > +			atomic_inc(&max_tr.data[cpu]->disabled);
> > +	}
> > +
> >  	if (val != global_trace.entries) {
> >  		ret = ring_buffer_resize(global_trace.buffer, val);
> >  		if (ret < 0) {
> > @@ -2735,6 +2743,13 @@ tracing_entries_write(struct file *filp, const char __user *ubuf,
> >  	if (tracing_disabled)
> >  		cnt = -ENOMEM;
> >   out:
> > +	for_each_tracing_cpu(cpu) {
> > +		if (global_trace.data[cpu])
> > +			atomic_dec(&global_trace.data[cpu]->disabled);
> > +		if (max_tr.data[cpu])
> > +			atomic_dec(&max_tr.data[cpu]->disabled);
> > +	}
> > +
> >  	max_tr.entries = global_trace.entries;
> >  	mutex_unlock(&trace_types_lock);
> >  
> > -- 
> > 1.5.6.5

-- 
Pekka Paalanen
http://www.iki.fi/pq/