linux-kernel - Re: [Patch] lkdtm: avoid calling lkdtm_do

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Message-Id: <201202011529.32778.arnd@arndb.de>
Date:	Wed, 1 Feb 2012 15:29:32 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Prarit Bhargava <prarit@...hat.com>,
	"Greg Kroah-Hartman" <greg@...ah.com>
Subject: Re: [Patch] lkdtm: avoid calling lkdtm_do_action() with spin lock held

On Wednesday 01 February 2012, Cong Wang wrote:
> On 01/31/2012 11:35 PM, Arnd Bergmann wrote:
> > On Tuesday 31 January 2012, Cong Wang wrote:
> >> @@ -323,14 +323,16 @@ static void lkdtm_do_action(enum ctype which)
> >>          }
> >>          case CT_WRITE_AFTER_FREE: {
> >>                  size_t len = 1024;
> >> -               u32 *data = kmalloc(len, GFP_KERNEL);
> >> +               u32 *data = kmalloc(len, GFP_ATOMIC);
> >>
> >>                  kfree(data);
> >> -               schedule();
> >> +               udelay(100);
> >>                  memset(data, 0x78, len);
> >>                  break;
> >>          }
> >
> > I can't think of why the udelay would have any positive effect here,
> > if the idea of the schedule was to let some other process allocate and
> > use the memory.
> 
> 
> Hmm, on SMP udelay on this CPU will give a chance to other CPU's to use 
> that memory, right?
> 

There is a small chance for that, but it's much less likely than it would
be using another process on the same CPU, plus it requires SMP.

	Arnd
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