From: "Paul E. McKenney" <paulmck@kernel.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Florian Weimer <fw@deneb.enyo.de>,
Linus Torvalds <torvalds@linux-foundation.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Segher Boessenkool <segher@kernel.crashing.org>,
Will Deacon <will@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
Andrea Parri <parri.andrea@gmail.com>,
Boqun Feng <boqun.feng@gmail.com>,
Nicholas Piggin <npiggin@gmail.com>,
David Howells <dhowells@redhat.com>,
j alglave <j.alglave@ucl.ac.uk>,
luc maranget <luc.maranget@inria.fr>, akiyks <akiyks@gmail.com>,
linux-toolchains <linux-toolchains@vger.kernel.org>,
linux-arch <linux-arch@vger.kernel.org>
Subject: Re: [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency
Date: Thu, 14 Oct 2021 09:14:30 -0700 [thread overview]
Message-ID: <20211014161430.GC880162@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <20211014021431.GA910341@rowland.harvard.edu>
On Wed, Oct 13, 2021 at 10:14:31PM -0400, Alan Stern wrote:
> On Wed, Oct 13, 2021 at 05:01:04PM -0700, Paul E. McKenney wrote:
> > 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@redhat.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?
> >
> > 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?
>
> Here's a easy cop-out. Imagine a version of READ_ONCE that is
> equivalent to:
>
> a normal READ_ONCE on TSO architectures,
>
> a load-acquire on more weakly ordered architectures.
>
> Call it READ_ONCE_FOR_COND, for the sake of argument. Then as long as
> people are careful to use READ_ONCE_FOR_COND when loading the values
> that a conditional expression depends on, and WRITE_ONCE for the
> important stores in the branches of the "if" statement, all
> architectures will have the desired ordering. (In fact, if there are
> multiple loads involved in the condition then only the last one has to
> be READ_ONCE_FOR_COND; the others can just be READ_ONCE.)
>
> Of course, this is not optimal on non-TSO archictecture. That's why I
> called it a cop-out. But at least it is simple and easy.
That is the ARMv8 approach in CONFIG_LTO=y kernels. ;-)
Thanx, Paul
next prev parent reply other threads:[~2021-10-14 16:14 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-28 21:15 [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency Mathieu Desnoyers
2021-09-29 12:06 ` Marco Elver
2021-10-01 15:45 ` Mathieu Desnoyers
2021-10-01 16:20 ` Linus Torvalds
2021-10-01 17:28 ` Mathieu Desnoyers
2021-10-01 18:18 ` Linus Torvalds
2021-09-29 12:28 ` Florian Weimer
2021-09-29 17:41 ` Segher Boessenkool
2021-09-29 19:46 ` Florian Weimer
2021-10-01 16:13 ` Mathieu Desnoyers
2021-10-01 16:26 ` Florian Weimer
2021-10-01 16:35 ` Linus Torvalds
2021-10-10 14:02 ` Florian Weimer
2021-10-14 0:01 ` Paul E. McKenney
2021-10-14 2:14 ` Alan Stern
2021-10-14 16:14 ` Paul E. McKenney [this message]
2021-10-14 15:58 ` Florian Weimer
2021-10-14 16:23 ` Paul E. McKenney
2021-10-14 18:19 ` Florian Weimer
2021-10-14 21:09 ` Paul E. McKenney
2021-10-14 22:36 ` Linus Torvalds
2021-09-30 13:28 ` Mathieu Desnoyers
2021-09-29 14:47 ` Linus Torvalds
2021-09-29 14:54 ` Linus Torvalds
2021-09-29 19:50 ` Mathieu Desnoyers
2021-09-29 20:13 ` Mathieu Desnoyers
2021-09-29 19:27 ` Mathieu Desnoyers
2021-09-29 22:14 ` Linus Torvalds
2021-09-29 21:47 ` Segher Boessenkool
2021-09-29 23:57 ` Paul E. McKenney
2021-10-01 15:28 ` Mathieu Desnoyers
2021-10-01 22:53 ` Paul E. McKenney
2021-10-01 19:10 ` Segher Boessenkool
2021-10-01 22:50 ` Paul E. McKenney
2021-10-02 14:29 ` Alan Stern
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