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Date: Tue, 26 Jan 2010 23:39:02 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
       Ingo Molnar <mingo@elte.hu>
Subject: Re: [PATCH 1/5] tracing: Prevent kernel oops with corrupted buffer
Message-ID: <20100126223900.GA5223@nowhere>
References: <20100126220923.534282809@goodmis.org> <20100126221712.447066697@goodmis.org> <20100126143223.e4332098.akpm@linux-foundation.org>
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On Tue, Jan 26, 2010 at 02:32:23PM -0800, Andrew Morton wrote:
> > +	if (WARN_ON_ONCE(pid < 0)) {
> > +		strcpy(comm, "<XXX>");
> > +		return;
> > +	}
> > +
> >  	if (pid > PID_MAX_DEFAULT) {
> >  		strcpy(comm, "<...>");
> >  		return;
> 
> But why is it WARN_ON_ONCE()?  That will only fix the problem a single
> time.  On the second occurrence, it will oops again.



The warning will be produced only once, but after that,
the condition is still checked like a simple if:

#define WARN_ON_ONCE(condition)	({				\
	static bool __warned;					\
	int __ret_warn_once = !!(condition);			\
								\
	if (unlikely(__ret_warn_once))				\
		if (WARN_ON(!__warned)) 			\
			__warned = true;			\
	unlikely(__ret_warn_once);				\
})


And since this function can be called anytime we have a trace
to print to the user, we don't want to encumber with thousands
of warnings.