From: Will Deacon <will@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
paulmck@kernel.org, stern@rowland.harvard.edu,
parri.andrea@gmail.com, boqun.feng@gmail.com, npiggin@gmail.com,
dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
akiyks@gmail.com, linux-kernel@vger.kernel.org,
linux-toolchains@vger.kernel.org, linux-arch@vger.kernel.org
Subject: Re: [RFC] LKMM: Add volatile_if()
Date: Fri, 4 Jun 2021 16:13:57 +0100 [thread overview]
Message-ID: <20210604151356.GC2793@willie-the-truck> (raw)
In-Reply-To: <YLoxAOua/qsZXNmY@hirez.programming.kicks-ass.net>
On Fri, Jun 04, 2021 at 03:56:16PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 04, 2021 at 02:44:22PM +0100, Will Deacon wrote:
> > On Fri, Jun 04, 2021 at 01:31:48PM +0200, Peter Zijlstra wrote:
> > > On Fri, Jun 04, 2021 at 11:44:00AM +0100, Will Deacon wrote:
> > > > On Fri, Jun 04, 2021 at 12:12:07PM +0200, Peter Zijlstra wrote:
> > >
> > > > > Usage of volatile_if requires the @cond to be headed by a volatile load
> > > > > (READ_ONCE() / atomic_read() etc..) such that the compiler is forced to
> > > > > emit the load and the branch emitted will have the required
> > > > > data-dependency. Furthermore, volatile_if() is a compiler barrier, which
> > > > > should prohibit the compiler from lifting anything out of the selection
> > > > > statement.
> > > >
> > > > When building with LTO on arm64, we already upgrade READ_ONCE() to an RCpc
> > > > acquire. In this case, it would be really good to avoid having the dummy
> > > > conditional branch somehow, but I can't see a good way to achieve that.
> > >
> > > #ifdef CONFIG_LTO
> > > /* Because __READ_ONCE() is load-acquire */
> > > #define volatile_cond(cond) (cond)
> > > #else
> > > ....
> > > #endif
> > >
> > > Doesn't work? Bit naf, but I'm thinking it ought to do.
> >
> > The problem is with relaxed atomic RMWs; we don't upgrade those to acquire
> > atm as they're written in asm, but we'd need volatile_cond() to work with
> > them. It's a shame, because we only have RCsc RMWs on arm64, so it would
> > be a bit more expensive.
>
> Urgh, I see. Compiler can't really help in that case either I'm afraid.
> They'll never want to modify loads that originate in an asm(). They'll
> say to use the C11 _Atomic crud.
Indeed. That's partly what led me down the route of thinking about "control
ordering" to sit between relaxed and acquire. So you have READ_ONCE_CTRL()
instead of this, but then we can't play your asm goto trick.
If we could push the memory access _and_ the branch down into the new
volatile_if helper, a bit like we do for smp_cond_load_*(), that would
help but it makes the thing a lot harder to use.
In fact, maybe it's actually necessary to bundle the load and branch
together. I looked at some of the examples of compilers breaking control
dependencies from memory-barriers.txt and the "boolean short-circuit"
example seems to defeat volatile_if:
void foo(int *x, int *y)
{
volatile_if (READ_ONCE(*x) || 1 > 0)
WRITE_ONCE(*y, 42);
}
Although we get a conditional branch emitted, it's headed by an immediate
move instruction and the result of the load is discarded:
38: d503233f paciasp
3c: b940001f ldr wzr, [x0]
40: 52800028 mov w8, #0x1 // #1
44: b5000068 cbnz x8, 50 <foo+0x18>
48: d50323bf autiasp
4c: d65f03c0 ret
50: d503249f bti j
54: 52800548 mov w8, #0x2a // #42
58: b9000028 str w8, [x1]
5c: d50323bf autiasp
60: d65f03c0 ret
Will
next prev parent reply other threads:[~2021-06-04 15:14 UTC|newest]
Thread overview: 128+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-04 10:12 [RFC] LKMM: Add volatile_if() Peter Zijlstra
2021-06-04 10:44 ` Will Deacon
2021-06-04 11:13 ` Will Deacon
2021-06-04 11:31 ` Peter Zijlstra
2021-06-04 13:44 ` Will Deacon
2021-06-04 13:56 ` Peter Zijlstra
2021-06-04 15:13 ` Will Deacon [this message]
2021-06-04 15:22 ` Peter Zijlstra
2021-06-04 15:36 ` Alan Stern
2021-06-04 15:42 ` Peter Zijlstra
2021-06-04 15:51 ` Alan Stern
2021-06-04 16:17 ` Peter Zijlstra
2021-06-04 18:27 ` Alan Stern
2021-06-04 19:09 ` Linus Torvalds
2021-06-04 19:18 ` Linus Torvalds
2021-06-04 20:56 ` Paul E. McKenney
2021-06-04 21:27 ` Linus Torvalds
2021-06-04 21:40 ` Paul E. McKenney
2021-06-04 22:19 ` Linus Torvalds
2021-06-05 14:57 ` Alan Stern
2021-06-06 0:14 ` Paul E. McKenney
2021-06-06 1:29 ` Alan Stern
2021-06-06 3:41 ` Linus Torvalds
2021-06-06 4:43 ` Paul E. McKenney
2021-06-06 13:17 ` Segher Boessenkool
2021-06-06 19:07 ` Paul E. McKenney
2021-06-06 12:59 ` Segher Boessenkool
2021-06-06 13:47 ` Alan Stern
2021-06-06 17:13 ` Segher Boessenkool
2021-06-06 18:25 ` Linus Torvalds
2021-06-06 19:19 ` Segher Boessenkool
2021-06-06 18:41 ` Alan Stern
2021-06-06 18:59 ` Jakub Jelinek
2021-06-06 19:15 ` Paul E. McKenney
2021-06-06 19:22 ` Linus Torvalds
2021-06-06 20:11 ` Segher Boessenkool
2021-06-06 21:19 ` Alexander Monakov
2021-06-06 22:38 ` Linus Torvalds
2021-06-06 23:39 ` Rasmus Villemoes
2021-06-06 23:44 ` Rasmus Villemoes
2021-06-07 8:01 ` Alexander Monakov
2021-06-07 8:27 ` Marco Elver
2021-06-07 15:28 ` Paul E. McKenney
2021-06-07 17:04 ` Marco Elver
2021-06-08 9:30 ` Marco Elver
2021-06-08 11:22 ` Peter Zijlstra
2021-06-08 15:28 ` Segher Boessenkool
2021-06-09 12:44 ` Marco Elver
2021-06-09 15:31 ` Segher Boessenkool
2021-06-09 16:13 ` Marco Elver
2021-06-09 17:14 ` Segher Boessenkool
2021-06-09 17:31 ` Nick Desaulniers
2021-06-09 20:24 ` Segher Boessenkool
2021-06-09 18:25 ` Linus Torvalds
2021-06-07 17:52 ` Segher Boessenkool
2021-06-07 18:07 ` Alexander Monakov
2021-06-07 18:18 ` Segher Boessenkool
2021-06-07 17:42 ` Segher Boessenkool
2021-06-07 20:31 ` Linus Torvalds
2021-06-07 22:54 ` Segher Boessenkool
2021-06-06 11:53 ` Segher Boessenkool
2021-06-06 13:45 ` Alan Stern
2021-06-06 18:04 ` Linus Torvalds
2021-06-06 18:22 ` Alan Stern
2021-06-06 18:43 ` Linus Torvalds
2021-06-07 10:43 ` Peter Zijlstra
2021-06-07 11:52 ` Will Deacon
2021-06-07 15:25 ` Paul E. McKenney
2021-06-07 16:02 ` Will Deacon
2021-06-07 18:08 ` Paul E. McKenney
2021-07-30 17:20 ` Jade Alglave
2021-07-30 20:35 ` Alan Stern
2021-08-02 21:18 ` Alan Stern
2021-08-02 23:31 ` Paul E. McKenney
2021-08-04 20:09 ` Alan Stern
2021-08-05 19:47 ` Alan Stern
2021-08-07 0:51 ` Alan Stern
2021-06-06 18:40 ` Segher Boessenkool
2021-06-06 18:48 ` Linus Torvalds
2021-06-06 18:53 ` Linus Torvalds
2021-06-06 19:52 ` Segher Boessenkool
2021-06-06 20:11 ` Linus Torvalds
2021-06-06 20:26 ` Segher Boessenkool
2021-06-06 23:37 ` Paul E. McKenney
2021-06-07 14:12 ` Segher Boessenkool
2021-06-07 15:27 ` Paul E. McKenney
2021-06-07 18:23 ` Segher Boessenkool
2021-06-07 19:51 ` Alan Stern
2021-06-07 20:16 ` Paul E. McKenney
2021-06-07 22:40 ` Segher Boessenkool
2021-06-07 23:26 ` Paul E. McKenney
2021-06-07 10:52 ` Peter Zijlstra
2021-06-07 14:16 ` Segher Boessenkool
2021-06-04 22:05 ` Peter Zijlstra
2021-06-05 3:14 ` Alan Stern
2021-06-05 16:24 ` Linus Torvalds
2021-06-04 15:50 ` Segher Boessenkool
2021-06-04 15:47 ` Segher Boessenkool
2021-06-04 11:44 ` Peter Zijlstra
2021-06-04 14:13 ` Paul E. McKenney
2021-06-04 15:35 ` Segher Boessenkool
2021-06-04 16:10 ` Peter Zijlstra
2021-06-04 16:40 ` Segher Boessenkool
2021-06-04 18:55 ` Paul E. McKenney
2021-06-04 19:53 ` Segher Boessenkool
2021-06-04 20:40 ` Paul E. McKenney
2021-06-06 11:36 ` Segher Boessenkool
2021-06-06 19:01 ` Paul E. McKenney
2021-06-04 14:25 ` Alan Stern
2021-06-04 16:09 ` Segher Boessenkool
2021-06-04 16:33 ` Peter Zijlstra
2021-06-04 16:30 ` Linus Torvalds
2021-06-04 16:37 ` Peter Zijlstra
2021-06-04 16:52 ` Segher Boessenkool
2021-06-04 17:10 ` Linus Torvalds
2021-06-04 17:24 ` Segher Boessenkool
2021-06-04 17:38 ` Linus Torvalds
2021-06-04 18:25 ` Segher Boessenkool
2021-06-04 19:17 ` Peter Zijlstra
2021-06-04 20:43 ` Paul E. McKenney
2021-06-04 18:23 ` Alan Stern
2021-06-08 12:48 ` David Laight
2021-09-24 18:38 ` Mathieu Desnoyers
2021-09-24 19:52 ` Alan Stern
2021-09-24 20:22 ` Mathieu Desnoyers
2021-09-24 19:55 ` Segher Boessenkool
2021-09-24 20:39 ` Mathieu Desnoyers
2021-09-24 22:07 ` Mathieu Desnoyers
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