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 14:44:22 +0100 [thread overview]
Message-ID: <20210604134422.GA2793@willie-the-truck> (raw)
In-Reply-To: <YLoPJDzlTsvpjFWt@hirez.programming.kicks-ass.net>
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.
> > > +/**
> > > + * volatile_if() - Provide a control-dependency
> > > + *
> > > + * volatile_if(READ_ONCE(A))
> > > + * WRITE_ONCE(B, 1);
> > > + *
> > > + * will ensure that the STORE to B happens after the LOAD of A. Normally a
> > > + * control dependency relies on a conditional branch having a data dependency
> > > + * on the LOAD and an architecture's inability to speculate STOREs. IOW, this
> > > + * provides a LOAD->STORE order.
> > > + *
> > > + * Due to optimizing compilers extra care is needed; as per the example above
> > > + * the LOAD must be 'volatile' qualified in order to ensure the compiler
> > > + * actually emits the load, such that the data-dependency to the conditional
> > > + * branch can be formed.
> > > + *
> > > + * Secondly, the compiler must be prohibited from lifting anything out of the
> > > + * selection statement, as this would obviously also break the ordering.
> > > + *
> > > + * Thirdly, and this is the tricky bit, architectures that allow the
> > > + * LOAD->STORE reorder must ensure the compiler actually emits the conditional
> > > + * branch instruction, this isn't possible in generic.
> > > + *
> > > + * See the volatile_cond() wrapper.
> > > + */
> > > +#define volatile_if(cond) if (volatile_cond(cond))
> >
> > The thing I really dislike about this is that, if the compiler _does_
> > emit a conditional branch for the C 'if', then we get a pair of branch
> > instructions in close proximity to each other which the predictor is likely
> > to hate. I wouldn't be surprised if an RCpc acquire heading the dependency
> > actually performs better on modern arm64 cores in the general case.
>
> jump_label / static_branch relies on asm goto inside if to get optimized
> away, so I'm fairly confident this will not result in a double branch,
> because yes, that would blow.
I gave it a spin and you're right. Neat!
Will
next prev parent reply other threads:[~2021-06-04 13:44 UTC|newest]
Thread overview: 127+ 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 [this message]
2021-06-04 13:56 ` Peter Zijlstra
2021-06-04 15:13 ` Will Deacon
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
[not found] ` <20210730172020.GA32396@knuckles.cs.ucl.ac.uk>
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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