linux-kernel - Re: call_function_many: fix list delete vs add race

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Date:	Tue, 01 Feb 2011 11:22:44 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Milton Miller <miltonm@....com>, akpm@...ux-foundation.org,
	Anton Blanchard <anton@...ba.org>,
	xiaoguangrong@...fujitsu.com, mingo@...e.hu, jaxboe@...ionio.com,
	npiggin@...il.com, rusty@...tcorp.com.au,
	torvalds@...ux-foundation.org, paulmck@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org
Subject: Re: call_function_many: fix list delete vs add race

On Mon, 2011-01-31 at 22:17 +0100, Peter Zijlstra wrote:
> That's wrong:
> 
>  ->foo =
>  LOCK
>  UNLOCK
>  ->bar =
> 
> can be re-ordered as:
> 
>  LOCK
>  ->bar =
>  ->foo =
>  UNLOCK

Can it ? I though UNLOCK had a write barrier semantic ? It does on power
at least :-) It should have since it shall prevent stores inside the
lock region to pass the store of the unlock itself anyways.

So yes, ->bar = can leak into the lock, as can ->foo =, but they can't
be re-ordered vs. each other because the implied barrier will keep ->foo
= in the same "domain" as the unlock itself.

Or do other archs do something really nasty here that don't provide this
guarantee ?

Cheers,
Ben.

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