linux-kernel - Re: [PATCH 1/2] kthread: add barriers to set_kthread_struct() and to

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Date:   Thu, 16 Mar 2017 11:33:01 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, Chris Mason <clm@...com>,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH 1/2] kthread: add barriers to set_kthread_struct() and
 to_kthread()

Hello, Oleg.

On Thu, Mar 16, 2017 at 03:54:36PM +0100, Oleg Nesterov wrote:
> On 03/15, Tejun Heo wrote:
> >
> > Until now, all to_kthread() users are interlocked with kthread
> > creation and there's no need to have explicit barriers when setting
> > the kthread pointer or dereferencing it.
> >
> > However, There is a race condition where userland can interfere with a
> > kthread while it's being initialized.  To close it, to_kthread() needs
> > to be used from an unsynchronized context.
> 
> So this is preparation for 2/2... IIUC, the current code is not buggy,
> just you need to add kthread_initialized() which can't work without
> this change.

Yeah, I could have been clearer.

> > +	/*
> > +	 * Paired with smp_wmb() in set_kthread_struct() and ensures that
> > +	 * the caller sees initialized content of the returned kthread.
> > +	 */
> > +	smp_read_barrier_depends();
> > +
> > +	return ptr;
> 
> This is almost off-topic, but I think lockless_dereference() will look
> better in to_kthread().
> 
> And perhaps we should add another helper, say,
> 
> 	#define lockless_assign_pointer(ptr, val)	\
> 		smp_store_release(&ptr, val)
> 
> for set_kthread_struct() ? it can have more users.
> 
> Not that I think you should change your patch, I am just asking.

Ah yeah, that would look better.  I vaguely remembered the new macro
but couldn't quite remember it fully. :)  Will update the patch.

Thanks.

-- 
tejun