linux-kernel - Re: current_thread_info() not respecting program order with gcc 4.8.x

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Message-ID: <528E94E2.3080102@ahsoftware.de>
Date:	Fri, 22 Nov 2013 00:18:58 +0100
From:	Alexander Holler <holler@...oftware.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Jakub Jelinek <jakub@...hat.com>,
	Richard Henderson <rth@...ddle.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Will Deacon <will.deacon@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	lttng-dev@...ts.lttng.org, Nathan Lynch <Nathan_Lynch@...tor.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Luis Lozano <llozano@...omium.org>,
	Bhaskar Janakiraman <bjanakiraman@...omium.org>,
	Han Shen <shenhan@...omium.org>
Subject: Re: current_thread_info() not respecting program order with gcc 4.8.x


Am 21.11.2013 23:32, schrieb Linus Torvalds:
> On Thu, Nov 21, 2013 at 8:02 AM, Alexander Holler <holler@...oftware.de> wrote:

> The bug is not that gcc can re-order or combine the accesses to "sp".
> WE WANT THAT TO HAPPEN.

Sure, and I don't disagree on that.

>
> The bug is *outside* that "current_thread_info()" macro/inline
> function. It's the *dereference* of the pointer that gcc re-orders.
> AND THAT IS WRONG.
>
> Gcc seems to mess up the alias analysis, and decide that the
> deferences cannot alias. Which is wrong. They clearly *can* alias,
> exactly because the value of "sp & ~(THREAD_SIZE - 1)" ends up having
> the same value all the time.

Sorry, that I still disagree.

I try to describe it more clearly why I still think that the problem 
might be because of that const declaration.

(...)

foobar1 = current_thread_info() __attribute_const__ {
	return sp->somewhere_local;
}

(...)

foobar2 = current_thread_info() __attribute_const__ {
	return sp->somewhere_local;
}

So, even if sp is the same in both cases, that const states that 
wherever sp points to is local to current_thread_info(), so it can't be 
the same for both cases.

Regards,

Alexander Holler
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