linux-kernel - Re: [RFC] LKMM: Add volatile

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Message-ID: <20210605145739.GB1712909@rowland.harvard.edu>
Date:   Sat, 5 Jun 2021 10:57:39 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nick Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-toolchains@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC] LKMM: Add volatile_if()

On Fri, Jun 04, 2021 at 03:19:11PM -0700, Linus Torvalds wrote:
> Now, part of this is that I do think that in *general* we should never
> use this very suble load-cond-store pattern to begin with. We should
> strive to use more smp_load_acquire() and smp_store_release() if we
> care about ordering of accesses. They are typically cheap enough, and
> if there's much of an ordering issue, they are the right things to do.
> 
> I think the whole "load-to-store ordering" subtle non-ordered case is
> for very very special cases, when you literally don't have a general
> memory ordering, you just have an ordering for *one* very particular
> access. Like some of the very magical code in the rw-semaphore case,
> or that smp_cond_load_acquire().
> 
> IOW, I would expect that we have a handful of uses of this thing. And
> none of them have that "the conditional store is the same on both
> sides" pattern, afaik.
> 
> And immediately when the conditional store is different, you end up
> having a dependency on it that orders it.
> 
> But I guess I can accept the above made-up example as an "argument",
> even though I feel it is entirely irrelevant to the actual issues and
> uses we have.

Indeed, the expansion of the currently proposed version of

	volatile_if (A) {
		B;
	} else {
		C;
	}

is basically the same as

	if (A) {
		barrier();
		B;
	} else {
		barrier();
		C;
	}

which is just about as easy to write by hand.  (For some reason my 
fingers don't like typing "volatile_"; the letters tend to get 
scrambled.)

So given that:

	1. Reliance on control dependencies is uncommon in the kernel,
	   and

	2. The loads in A could just be replaced with load_acquires
	   at a low penalty (or store-releases could go into B and C),

it seems that we may not need volatile_if at all!  The only real reason 
for having it in the first place was to avoid the penalty of 
load-acquire on architectures where it has a significant cost, when the 
control dependency would provide the necessary ordering for free.  Such 
architectures are getting less and less common.

Alan