linux-kernel - Re: [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency

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Date:   Thu, 14 Oct 2021 17:58:16 +0200
From:   Florian Weimer <fw@...eb.enyo.de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        j alglave <j.alglave@....ac.uk>,
        luc maranget <luc.maranget@...ia.fr>,
        akiyks <akiyks@...il.com>,
        linux-toolchains <linux-toolchains@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency

* Paul E. McKenney:

> On Sun, Oct 10, 2021 at 04:02:02PM +0200, Florian Weimer wrote:
>> * Linus Torvalds:
>> 
>> > On Fri, Oct 1, 2021 at 9:26 AM Florian Weimer <fweimer@...hat.com> wrote:
>> >>
>> >> Will any conditional branch do, or is it necessary that it depends in
>> >> some way on the data read?
>> >
>> > The condition needs to be dependent on the read.
>> >
>> > (Easy way to see it: if the read isn't related to the conditional or
>> > write data/address, the read could just be delayed to after the
>> > condition and the store had been done).
>> 
>> That entirely depends on how the hardware is specified to work.  And
>> the hardware could recognize certain patterns as always producing the
>> same condition codes, e.g., AND with zero.  Do such tests still count?
>> It depends on what the specification says.
>> 
>> What I really dislike about this: Operators like & and < now have side
>> effects, and is no longer possible to reason about arithmetic
>> expressions in isolation.
>
> Is there a reasonable syntax that might help with these issues?

Is this really a problem of syntax?

> Yes, I know, we for sure have conflicting constraints on "reasonable"
> on copy on this email.  What else is new?  ;-)
>
> I could imagine a tag of some sort on the load and store, linking the
> operations that needed to be ordered.  You would also want that same
> tag on any conditional operators along the way?  Or would the presence
> of the tags on the load and store suffice?

If the load is assigned to a local variable whose address is not taken
and which is only assigned this once, it could be used to label the
store.  Then the compiler checks if all paths from the load to the
store feature a condition that depends on the local variable (where
qualifying conditions probably depend on the architecture).  If it
can't prove that is the case, it emits a fake no-op condition that
triggers the hardware barrier.  This formulation has the advantage
that it does not add side effects to operators like <.  It even
generalizes to different barrier-implying instructions besides
conditional branches.

But I'm not sure if all this complexity will be a tangible improvement
over just using that no-op condition all the time (whether implied by
READ_ONCE, or in a separate ctrl_dep macro).