linux-kernel - Re: [PATCH tracing/kprobes 2/6] ftrace: Fix trace_add_event

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Message-Id: <1253111371.20020.198.camel@gandalf.stny.rr.com>
Date:	Wed, 16 Sep 2009 10:29:31 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	lkml <linux-kernel@...r.kernel.org>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>,
	Jim Keniston <jkenisto@...ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Christoph Hellwig <hch@...radead.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Jason Baron <jbaron@...hat.com>,
	"K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Tom Zanussi <tzanussi@...il.com>
Subject: Re: [PATCH tracing/kprobes 2/6] ftrace: Fix trace_add_event_call()
 to initialize list

On Wed, 2009-09-16 at 10:17 -0400, Masami Hiramatsu wrote:
> Steven Rostedt wrote:

> >> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> >> index ba34920..38e82a5 100644
> >> --- a/kernel/trace/trace_events.c
> >> +++ b/kernel/trace/trace_events.c
> >> @@ -1009,10 +1009,14 @@ static int __trace_add_event_call(struct ftrace_event_call *call)
> >>  	if (!d_events)
> >>  		return -ENOENT;
> >>  
> >> +	INIT_LIST_HEAD(&call->list);
> > 
> > The INIT_LIST_HEAD is not needed here. The list_add will assign it.
> 
> Without initializing it, list debugging code warns always :-)
> Please see, __list_add()@lib/list_debug.c

/me looks

from: include/linux/list.h

static inline void list_add(struct list_head *new, struct list_head *head)
{
	__list_add(new, head, head->next);
}

from: lib/list_debug.c

void __list_add(struct list_head *new,
			      struct list_head *prev,
			      struct list_head *next)
{
	WARN(next->prev != prev,
		"list_add corruption. next->prev should be "
		"prev (%p), but was %p. (next=%p).\n",
		prev, next->prev, next);
	WARN(prev->next != next,
		"list_add corruption. prev->next should be "
		"next (%p), but was %p. (prev=%p).\n",
		next, prev->next, prev);
	next->prev = new;
	new->next = next;
	new->prev = prev;
	prev->next = new;
}

What you pass in is:

        list_add(&call->list, &ftrace_events);

new = &call->list;
prev = &ftrace_events->prev;
next = &ftrace_events->next;

The above code never tests "new". The INIT_LIST_HEAD is useless.

-- Steve


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