linux-kernel - Re: [RFC PATCH] kmemleak: Scan all thread stacks

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Message-ID: <20090722201638.GI6757@linux.vnet.ibm.com>
Date:	Wed, 22 Jul 2009 13:16:38 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] kmemleak: Scan all thread stacks

On Wed, Jul 22, 2009 at 06:03:29PM +0100, Catalin Marinas wrote:
> On Wed, 2009-07-22 at 09:18 -0700, Paul E. McKenney wrote:
> > On Fri, Jul 17, 2009 at 07:01:09PM +0200, Peter Zijlstra wrote:
> > > On Fri, 2009-07-17 at 17:57 +0100, Catalin Marinas wrote:
> > > > On Fri, 2009-07-17 at 18:43 +0200, Ingo Molnar wrote:
> > > > > * Catalin Marinas <catalin.marinas@....com> wrote:
> > > > > > 2. Is it safe to use rcu_read_lock() and task_lock() when scanning the
> > > > > >    corresponding kernel stack (thread_info structure)? The loop doesn't
> > > > > >    do any modification to the task list. The reason for this is to
> > > > > >    allow kernel preemption when scanning the stacks.
> > > > > 
> > > > > you cannot generally preempt while holding the RCU read-lock.
> > > > 
> > > > This may work with rcupreempt enabled. But, with classic RCU is it safe
> > > > to call schedule (or cond_resched) while holding the RCU read-lock?
> > > 
> > > No.
> > 
> > What Peter said!  ;-)
> > 
> > However, you might be able to use SRCU (http://lwn.net/Articles/202847/),
> > which does allow blocking within read-side critical sections.
> 
> Thanks for the suggestion. But this would mean that the task_struct
> creation/deletion code should use the SRCU as well which I wouldn't
> modify. I'm also not entirely sure this could replace
> read_lock(&tasklist_lock)/read_unlock (as per the initial question).
> 
> The simplest fix for kmemleak is to not traverse the task list at all -
> http://lkml.org/lkml/2009/7/20/55. The patch is just like any other
> kmemleak annotation in the kernel.

Even better, agreed!

							Thanx, Paul
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