linux-kernel - Re: [PATCH 1/2] kthread: add a missing memory barrier to kthread

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Message-ID: <b647ffbd0803021428h277ed1e0q6d302f0ae9392346@mail.gmail.com>
Date:	Sun, 2 Mar 2008 23:28:35 +0100
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	"Rusty Russell" <rusty@...tcorp.com.au>
Cc:	"Andrew Morton" <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	"Nick Piggin" <nickpiggin@...oo.com.au>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	"Andy Whitcroft" <apw@...dowen.org>
Subject: Re: [PATCH 1/2] kthread: add a missing memory barrier to kthread_stop()

On 02/03/2008, Rusty Russell <rusty@...tcorp.com.au> wrote:
> On Thursday 21 February 2008 08:36:30 Dmitry Adamushko wrote:
>  > From: Dmitry Adamushko <dmitry.adamushko@...il.com>
>  > Subject: kthread: add a missing memory barrier to kthread_stop()
>  >
>  > We must ensure that kthread_stop_info.k has been updated before
>  > kthread's wakeup. This is required to properly support
>  > the use of kthread_should_stop() in the main loop of kthread.
>  >
>  > wake_up_process() doesn't imply a full memory barrier,
>  > so we add an explicit one.
>
>
> I always believed that wake_up_process() implies a write barrier.  It's pretty
>  common to set something up then wake the intended recipient.

No, it didn't imply a write mb. Moreover, the situation I tried to
address required a full mb (write vs. read ops. synchronization).

Please take a look at the discussion here http://lkml.org/lkml/2008/2/23/238

and here is a more detailed description of the problem :
http://groups.google.com/group/fa.linux.kernel/browse_thread/thread/44c45685680585fc/e58785df0eeee6f8?lnk=raot

>
>  So I think this patch is overkill, but I'm happy to be corrected.

We ended up adding an explicit smp_wmb() (a patch by Linus) to the
very beginning of try_to_wake_up(). Combined with the following
spin_lock() it acts as a 'full' mb for write vs. read ('read' takes
place inside try_to_wake_up()) and that's exactly what we need here.

The 1st link mentioned above may provide more details for interested readers.

So yes, now that we have got smp_wmb() in try_to_wake_up() this patch
is redundant.


>
>  Thanks!
>
> Rusty.
>


-- 
Best regards,
Dmitry Adamushko
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