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

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Date:	Thu, 21 Nov 2013 15:45:35 -0800
From:	Luis Lozano <llozano@...gle.com>
To:	Alexander Holler <holler@...oftware.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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>,
	Bhaskar Janakiraman <bjanakiraman@...omium.org>,
	Han Shen <shenhan@...omium.org>
Subject: Re: current_thread_info() not respecting program order with gcc 4.8.x

I think we need a reproducer. Without this we may all be going on the
wrong path. This whole conversation started on an *assumption* that
some accesses were being reordered.

evidence of the reorder or reproducer please?

Luis


On Thu, Nov 21, 2013 at 3:18 PM, Alexander Holler <holler@...oftware.de> wrote:
>
> 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



-- 

Luis A. Lozano | Software Engineer | llozano@...gle.com | +1 (408)431-5164
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