linux-kernel - Re: [PATCH 2/2] tracing - fix recursive user stack trace

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Date:	Thu, 11 Nov 2010 16:57:04 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Jiri Olsa <jolsa@...hat.com>, mingo@...e.hu, andi@...stfloor.org,
	lwoodman@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] tracing - fix recursive user stack trace

On Thu, 2010-11-11 at 08:13 +0800, Li Zefan wrote:
> Jiri Olsa wrote:
> > The user stack trace can fault when examining the trace. Which
> > would call the do_page_fault handler, which would trace again,
> > which would do the user stack trace, which would fault and call
> > do_page_fault again ...
> > 
> > Thus this is causing a recursive bug. We need to have a recursion
> > detector here.
> > 
> 
> I guess this is from what I reported to Redhat, triggered by
> the ftrace stress test. ;)
> 
> This patch should be the first patch, otherwise you introduce
> a regression. Though it merely a problem in this case, better
> avoid it.

Yeah, this should go into urgent, and the other patch can be queued for
38.


> 
> A nitpick below:
> 
> > 
> > Signed-off-by: Steven Rostedt <srostedt@...hat.com>
> > Signed-off-by: Jiri Olsa <jolsa@...hat.com>
> > ---
> >  kernel/trace/trace.c |   19 +++++++++++++++++++
> >  1 files changed, 19 insertions(+), 0 deletions(-)
> > 
> > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > index 82d9b81..0215e87 100644
> > --- a/kernel/trace/trace.c
> > +++ b/kernel/trace/trace.c
> > @@ -1284,6 +1284,8 @@ void trace_dump_stack(void)
> >  	__ftrace_trace_stack(global_trace.buffer, flags, 3, preempt_count());
> >  }
> >  
> > +static DEFINE_PER_CPU(int, user_stack_count);
> > +
> >  void
> >  ftrace_trace_userstack(struct ring_buffer *buffer, unsigned long flags, int pc)
> >  {
> > @@ -1302,6 +1304,18 @@ ftrace_trace_userstack(struct ring_buffer *buffer, unsigned long flags, int pc)
> >  	if (unlikely(in_nmi()))
> >  		return;
> >  
> > +	/*
> > +	 * prevent recursion, since the user stack tracing may
> > +	 * trigger other kernel events.
> > +	 */
> > +	preempt_disable();
> > +	if (__get_cpu_var(user_stack_count))
> > +		goto out;
> > +
> > +	__get_cpu_var(user_stack_count)++;
> > +
> > +
> > +
> 
> redundant blank lines.

I can pull this patch with the fix.

Thanks!

-- Steve

> 
> >  	event = trace_buffer_lock_reserve(buffer, TRACE_USER_STACK,
> >  					  sizeof(*entry), flags, pc);
> >  	if (!event)
> > @@ -1319,6 +1333,11 @@ ftrace_trace_userstack(struct ring_buffer *buffer, unsigned long flags, int pc)
> >  	save_stack_trace_user(&trace);
> >  	if (!filter_check_discard(call, entry, buffer, event))
> >  		ring_buffer_unlock_commit(buffer, event);
> > +
> > +	__get_cpu_var(user_stack_count)--;
> > +
> > + out:
> > +	preempt_enable();
> >  }
> >  
> >  #ifdef UNUSED


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