linux-kernel - Re: [PATCH] documentation: explain memory barriers

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Message-ID: <48ED9BFB.4060904@redhat.com>
Date:	Thu, 09 Oct 2008 01:51:55 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Randy Dunlap <randy.dunlap@...cle.com>,
	Ben Hutchings <bhutchings@...arflare.com>,
	Mikulas Patocka <mpatocka@...hat.com>,
	linux-kernel@...r.kernel.org, linux-mm@...r.kernel.org
Subject: Re: [PATCH] documentation: explain memory barriers

Andrew Morton wrote:
> On Wed, 08 Oct 2008 21:17:58 -0400 Chris Snook <csnook@...hat.com> wrote:
> 
>> Randy Dunlap wrote:
>>> On Wed, 1 Oct 2008 22:54:04 -0700 Andrew Morton wrote:
>>>
>>>> This sequence is repeated three or four times and should be pulled out
>>>> into a well-commented function.  That comment should explain the logic
>>>> behind the use of these barriers, please.
>>> and on 2008-OCT-08 Ben Hutchings wrote:
>>>
>>>> All memory barriers need a comment to explain why and what they're doing.
> 
> I approve this message.
> 
>> Seriously?  When a barrier is used, it's generally self-evident what 
>> it's doing.
> 
> fs/buffer.c:sync_buffer().  Have fun.

The real disaster there is the clear_buffer_##name macro and friends, as
evidenced by fs/ext2/inode.c:599

                clear_buffer_new(bh_result); /* What's this do? */

I'm completely in favor of documenting everything that can potentially interact
with that train wreck, but I maintain that the vast majority of memory barriers
are self-evident.

-- Chris
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