From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: umgwanakikbuti@gmail.com, mingo@elte.hu, ktkhai@parallels.com,
rostedt@goodmis.org, tglx@linutronix.de, juri.lelli@gmail.com,
pang.xunlei@linaro.org, oleg@redhat.com,
wanpeng.li@linux.intel.com, linux-kernel@vger.kernel.org,
Al Viro <viro@ZenIV.linux.org.uk>,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 11/18] seqcount: Introduce raw_write_seqcount_barrier()
Date: Thu, 18 Jun 2015 11:15:05 +0200 [thread overview]
Message-ID: <20150618091505.GI19282@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20150617180214.GJ3913@linux.vnet.ibm.com>
On Wed, Jun 17, 2015 at 11:02:14AM -0700, Paul E. McKenney wrote:
> On Wed, Jun 17, 2015 at 07:11:40PM +0200, Peter Zijlstra wrote:
> > On Wed, Jun 17, 2015 at 09:37:31AM -0700, Paul E. McKenney wrote:
> > > The point of std::atomic<> (and of the equivalent C11 syntax) is to
> > > force the compiler to suppress optimizations that are unsafe for shared
> > > variables. We get more or less the same effect with volatile, protests
> > > from compiler people notwithstanding.
> > >
> > > I often tell the compiler guys that they have to expect make -some-
> > > concessions for being 30 years late to the concurrency party, but
> > > it nevertheless makes sense to future-proof our code where it is
> > > reasonable to do so.
> >
> > Right, so in that regards I would request the compiler option (and or
> > #pragma) that disables all the out-of-thin-air nonsense.
>
> OK. What is the form of the #pragma? If it focuses on a specific
> access, we are likely to get a lot of pushback.
I didn't have anything specific in mind; other than
#pragma no_speculative_stores_ever
Which would forbid all these retarded 'optimizations' for the entire
translation unit.
> > Because while they hide behind their undefined behaviour, the fact is
> > that all of their machines for the past 30 odd years have been relying
> > on this 'undefined' behaviour to work. This being the machines they've
> > been typing their useless specs on :-)
>
> Maybe I can scare them into doing all their work on UP systems. ;-)
>
> Interestingly enough, LLVM is taking a slightly different approach.
> Rather than invoke undefined behavior, they say that data races result
> in random bits being loaded. Not that it makes much difference to the
> health and well-being of the software, mind you...
I'm not sure I follow that argument.
> > I doubt there's a single OS kernel (that supports SMP configurations)
> > that does not rely on a whole host of 'undefined' behaviour.
>
> An alternative approach would be a compiler switch (or similar) that
> changed the default atomic access from SC to relaxed. Then shared
> variables could be marked atomic, and normal C code could be used to
> access them, but without the compiler emitting memory barriers all over
> the place (yes, even on x86).
See, I don;'t think that is a realistic approach. Who is going to audit
our ~16 million lines of code to mark all shared variables? Or all the
other existing code bases that rely on this behaviour?
next prev parent reply other threads:[~2015-06-18 9:15 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-11 12:46 [PATCH 00/18] sched: balance callbacks v4 Peter Zijlstra
2015-06-11 12:46 ` [PATCH 01/18] sched: Replace post_schedule with a balance callback list Peter Zijlstra
2015-06-11 15:32 ` Kirill Tkhai
2015-06-18 23:00 ` [tip:sched/hrtimers] " tip-bot for Peter Zijlstra
2015-06-11 12:46 ` [PATCH 02/18] sched: Use replace normalize_task() with __sched_setscheduler() Peter Zijlstra
2015-06-18 23:00 ` [tip:sched/hrtimers] " tip-bot for Peter Zijlstra
2015-06-11 12:46 ` [PATCH 03/18] sched: Allow balance callbacks for check_class_changed() Peter Zijlstra
2015-06-18 23:01 ` [tip:sched/hrtimers] " tip-bot for Peter Zijlstra
2015-06-11 12:46 ` [PATCH 04/18] sched,rt: Remove return value from pull_rt_task() Peter Zijlstra
2015-06-18 23:01 ` [tip:sched/hrtimers] " tip-bot for Peter Zijlstra
2015-06-11 12:46 ` [PATCH 05/18] sched,rt: Convert switched_{from,to}_rt() / prio_changed_rt() to balance callbacks Peter Zijlstra
2015-06-18 23:01 ` [tip:sched/hrtimers] sched, rt: Convert switched_{from, to}_rt() " tip-bot for Peter Zijlstra
2015-06-11 12:46 ` [PATCH 06/18] sched,dl: Remove return value from pull_dl_task() Peter Zijlstra
2015-06-18 23:02 ` [tip:sched/hrtimers] " tip-bot for Peter Zijlstra
2015-06-11 12:46 ` [PATCH 07/18] sched,dl: Convert switched_{from,to}_dl() / prio_changed_dl() to balance callbacks Peter Zijlstra
2015-06-18 23:02 ` [tip:sched/hrtimers] sched, dl: Convert switched_{from, to}_dl() " tip-bot for Peter Zijlstra
2015-06-11 12:46 ` [PATCH 08/18] hrtimer: Remove HRTIMER_STATE_MIGRATE Peter Zijlstra
2015-06-18 22:18 ` [tip:timers/core] " tip-bot for Oleg Nesterov
2015-06-11 12:46 ` [PATCH 09/18] hrtimer: Fix hrtimer_is_queued() hole Peter Zijlstra
2015-06-18 22:18 ` [tip:timers/core] " tip-bot for Peter Zijlstra
2015-06-11 12:46 ` [PATCH 10/18] seqcount: Rename write_seqcount_barrier() Peter Zijlstra
2015-06-18 22:19 ` [tip:timers/core] " tip-bot for Peter Zijlstra
2015-06-11 12:46 ` [PATCH 11/18] seqcount: Introduce raw_write_seqcount_barrier() Peter Zijlstra
2015-06-11 15:33 ` Paul E. McKenney
2015-06-11 21:45 ` Paul E. McKenney
2015-06-12 7:08 ` Peter Zijlstra
2015-06-12 18:59 ` Oleg Nesterov
2015-06-17 12:29 ` Peter Zijlstra
2015-06-17 14:57 ` Paul E. McKenney
2015-06-17 15:11 ` Peter Zijlstra
2015-06-17 15:42 ` Paul E. McKenney
2015-06-17 16:58 ` Peter Zijlstra
2015-06-17 15:49 ` Peter Zijlstra
2015-06-17 16:37 ` Paul E. McKenney
2015-06-17 17:11 ` Peter Zijlstra
2015-06-17 18:02 ` Paul E. McKenney
2015-06-18 9:15 ` Peter Zijlstra [this message]
2015-06-18 9:40 ` Ingo Molnar
2015-06-18 10:40 ` Peter Zijlstra
2015-06-18 16:54 ` Paul E. McKenney
2015-06-18 17:10 ` Steven Rostedt
2015-06-18 17:51 ` Paul E. McKenney
2015-06-18 22:19 ` [tip:timers/core] seqcount: Introduce raw_write_seqcount_barrier( ) tip-bot for Peter Zijlstra
2015-06-11 12:46 ` [PATCH 12/18] hrtimer: Allow hrtimer::function() to free the timer Peter Zijlstra
2015-06-18 22:19 ` [tip:timers/core] " tip-bot for Peter Zijlstra
2015-06-11 12:46 ` [PATCH 13/18] sched,dl: Fix sched class hopping CBS hole Peter Zijlstra
2015-06-18 23:02 ` [tip:sched/hrtimers] " tip-bot for Peter Zijlstra
2015-06-11 12:46 ` [PATCH 14/18] sched: Move code around Peter Zijlstra
2015-06-18 23:02 ` [tip:sched/hrtimers] " tip-bot for Peter Zijlstra
2015-06-11 12:46 ` [PATCH 15/18] sched: Streamline the task migration locking a little Peter Zijlstra
2015-06-18 23:03 ` [tip:sched/hrtimers] " tip-bot for Peter Zijlstra
2015-06-11 12:46 ` [PATCH 16/18] lockdep: Simplify lock_release() Peter Zijlstra
2015-06-18 23:03 ` [tip:sched/hrtimers] " tip-bot for Peter Zijlstra
2015-06-11 12:46 ` [PATCH 17/18] lockdep: Implement lock pinning Peter Zijlstra
2015-06-18 23:03 ` [tip:sched/hrtimers] " tip-bot for Peter Zijlstra
2015-06-11 12:46 ` [PATCH 18/18] sched,lockdep: Employ " Peter Zijlstra
2015-06-18 23:04 ` [tip:sched/hrtimers] " tip-bot for Peter Zijlstra
2015-12-29 5:41 ` [PATCH 00/18] sched: balance callbacks v4 Byungchul Park
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