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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

  reply	other threads:[~2021-06-04 13:44 UTC|newest]

Thread overview: 121+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-04 10:12 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

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