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

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Message-ID: <20150520151200.GS6776@linux.vnet.ibm.com>
Date:	Wed, 20 May 2015 08:12:00 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ramana Radhakrishnan <ramana.radhakrishnan@....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 Wed, May 20, 2015 at 03:15:48PM +0100, Ramana Radhakrishnan wrote:
> 
> 
> 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.

Agreed, there needs to be a memory_order_consume load up there somewhere.
(AKA rcu_dereference().)

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

Ah, I was assuming between x and z.  David, what was your intent?  ;-)

> >Or am I missing something subtle here?
> 
> No, it sounds like we are on the same page here.

Whew!  ;-)

							Thanx, Paul

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