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Subject: Re: [PATCH 2/5] mm: remove unlikly NULL from kfree
From: Pekka Enberg <penberg@cs.helsinki.fi>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
       Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>,
       Peter Zijlstra <peterz@infradead.org>,
       Roland McGrath <roland@redhat.com>,
       Nick Piggin <nickpiggin@yahoo.com.au>,
       Steven Rostedt <srostedt@redhat.com>,
       Christoph Lameter <cl@linux-foundation.org>,
       Matt Mackall <mpm@selenic.com>
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Date: Wed, 25 Mar 2009 10:01:51 +0200
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Hi Thomas,

On Wed, Mar 25, 2009 at 7:19 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
> > > This makes sense, since we now encourage developers to just call kfree
> > > without checking for NULL.

On Wed, 25 Mar 2009, Pekka Enberg wrote: 
> > But those are _error handling paths_ (at least supposed to be). I
> > wonder which call-sites are responsible for this. Can frtrace help us
> > here?

On Wed, 2009-03-25 at 08:50 +0100, Thomas Gleixner wrote:
> Why is this an error handler. We replaced tons of 
> 
>     if (obj)
>        kfree(obj);
> 
> constructs all over the kernel with kfree(obj); and let kfree deal
> with the NULL pointer.

We encourage developers not to check for kfree() in the common
out-of-memory error handling paths. But what Steven's results suggest is
that the common case is something like this:

   void *p = NULL;

   if (/* unlikely condition */)
       p = kmalloc(...);

   kfree(p);

which, quite frankly, doesn't make much sense to me. That's why I would
really want to know which call-sites are causing this before applying
the patch.

			Pekka