linux-kernel - Re: Memory allocator semantics

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Date:	Tue, 11 Feb 2014 15:20:01 +0200
From:	Pekka Enberg <penberg@...nel.org>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	Christoph Lameter <cl@...ux.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Matt Mackall <mpm@...enic.com>
Subject: Re: Memory allocator semantics

On Tue, Feb 11, 2014 at 2:14 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> In contrast, from kfree() to a kmalloc() returning some of the kfree()ed
> memory, I believe the kfree()/kmalloc() implementation must do any needed
> synchronization and ordering.  But that is a different set of examples,
> for example, this one:
>
>         CPU 0                   CPU 1
>         p->a = 42;              q = kmalloc(...); /* returning p */
>         kfree(p);               q->a = 5;
>                                 BUG_ON(q->a != 5);
>
> Unlike the situation with (A), (B), and (C), in this case I believe
> that it is kfree()'s and kmalloc()'s responsibility to ensure that
> the BUG_ON() never triggers.
>
> Make sense?

I'm not sure...

It's the caller's responsibility not to touch "p" after it's handed over to
kfree() - otherwise that's a "use-after-free" error.  If there's some reordering
going on here, I'm tempted to blame the caller for lack of locking.

                           Pekka
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