linux-kernel - Re: [RFC][PATCH 0/5] arch: atomic rework

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Date:	Thu, 27 Feb 2014 11:47:08 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	Torvald Riegel <triegel@...hat.com>,
	Will Deacon <will.deacon@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
	David Howells <dhowells@...hat.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"gcc@....gnu.org" <gcc@....gnu.org>
Subject: Re: [RFC][PATCH 0/5] arch: atomic rework

On Thu, Feb 27, 2014 at 11:06 AM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
>
> 3.      The comparison was against another RCU-protected pointer,
>         where that other pointer was properly fetched using one
>         of the RCU primitives.  Here it doesn't matter which pointer
>         you use.  At least as long as the rcu_assign_pointer() for
>         that other pointer happened after the last update to the
>         pointed-to structure.
>
> I am a bit nervous about #3.  Any thoughts on it?

I think that it might be worth pointing out as an example, and saying
that code like

   p = atomic_read(consume);
   X;
   q = atomic_read(consume);
   Y;
   if (p == q)
        data = p->val;

then the access of "p->val" is constrained to be data-dependent on
*either* p or q, but you can't really tell which, since the compiler
can decide that the values are interchangeable.

I cannot for the life of me come up with a situation where this would
matter, though. If "X" contains a fence, then that fence will be a
stronger ordering than anything the consume through "p" would
guarantee anyway. And if "X" does *not* contain a fence, then the
atomic reads of p and q are unordered *anyway*, so then whether the
ordering to the access through "p" is through p or q is kind of
irrelevant. No?

                     Linus
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