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Re: Compilers and RCU readers: Once more unto the breach!
- From: "Paul E. McKenney" <paulmck at linux dot vnet dot ibm dot com>
- To: Ramana Radhakrishnan <ramana dot radhakrishnan at arm dot com>
- Cc: David Howells <dhowells at redhat dot com>, Will Deacon <Will dot Deacon at arm dot com>, Linus Torvalds <torvalds at linux-foundation dot org>, Linux Kernel Mailing List <linux-kernel at vger dot kernel dot org>, "c++std-parallel at accu dot org" <c++std-parallel at accu dot org>, "linux-arch at vger dot kernel dot org" <linux-arch at vger dot kernel dot org>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>, p796231 <Peter dot Sewell at cl dot cam dot ac dot uk>, "mark dot batty at cl dot cam dot ac dot uk" <Mark dot Batty at cl dot cam dot ac dot uk>, Peter Zijlstra <peterz at infradead dot org>, Andrew Morton <akpm at linux-foundation dot org>, Ingo Molnar <mingo at kernel dot org>, "michaelw at ca dot ibm dot com" <michaelw at ca dot ibm dot com>
- Date: Wed, 20 May 2015 08:12:00 -0700
- Subject: Re: Compilers and RCU readers: Once more unto the breach!
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- Reply-to: paulmck at linux dot vnet dot ibm dot com
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@linux.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