linux-kernel - Re: Compilers and RCU readers: Once more unto the breach!

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Message-ID: <555C9714.6020001@arm.com>
Date:	Wed, 20 May 2015 15:15:48 +0100
From:	Ramana Radhakrishnan <ramana.radhakrishnan@....com>
To:	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>
CC:	David Howells <dhowells@...hat.com>,
	Will Deacon <Will.Deacon@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"c++std-parallel@...u.org" <c++std-parallel@...u.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"gcc@....gnu.org" <gcc@....gnu.org>,
	p796231 <Peter.Sewell@...cam.ac.uk>,
	"mark.batty@...cam.ac.uk" <Mark.Batty@...cam.ac.uk>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	"michaelw@...ibm.com" <michaelw@...ibm.com>
Subject: Re: Compilers and RCU readers: Once more unto the breach!



On 20/05/15 15:03, Paul E. McKenney wrote:
> On Wed, May 20, 2015 at 02:44:30PM +0100, Ramana Radhakrishnan wrote:
>>
>>
>> On 20/05/15 14:37, David Howells wrote:
>>> Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
>>>
>>>> I was thinking of "y" as a simple variable, but if it is something more
>>>> complex, then the compiler could do this, right?
>>>>
>>>> 	char *x;
>>>>
>>>> 	y;
>>>> 	x = z;
>>>
>>> Yeah.  I presume it has to maintain the ordering, though.
>>
>> The scheduler for e.g. is free to reorder if it can prove there is
>> no dependence (or indeed side-effects for y) between insns produced
>> for y and `x = z'.
>
> So for example, if y is independent of z, the compiler can do the
> following:
>
> 	char *x;
>
> 	x = z;
> 	y;
>
> But the dependency ordering is still maintained from z to x, so this
> is not a problem.


Well, reads if any of x (assuming x was initialized elsewhere) would 
need to happen before x got assigned to z.

I understood the original "maintain the ordering" as between the 
statements `x = z' and `y'.


>
> Or am I missing something subtle here?

No, it sounds like we are on the same page here.

regards
Ramana

>
> 							Thanx, Paul
>
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