From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Ingo Molnar <mingo@redhat.com>, Paul Turner <commonly@gmail.com>,
Andi Kleen <andi@firstfloor.org>, Chris Lameter <cl@linux.com>,
Dave Watson <davejwatson@fb.com>,
Josh Triplett <josh@joshtriplett.org>,
Linux API <linux-api@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Andrew Hunter <ahh@google.com>,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC PATCH 0/3] restartable sequences v2: fast user-space percpu critical sections
Date: Fri, 8 Apr 2016 08:41:36 +0200 [thread overview]
Message-ID: <20160408064136.GJ3448@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <CALCETrVGo1Di3qamxx1NAFUSN_o=-HnYRDpeVp7zrQEBwe5u-g@mail.gmail.com>
On Thu, Apr 07, 2016 at 09:43:33AM -0700, Andy Lutomirski wrote:
> enter the critical section:
> 1:
> movq %[cpu], %%r12
> movq {address of counter for our cpu}, %%r13
> movq {some fresh value}, (%%r13)
> cmpq %[cpu], %%r12
> jne 1b
This is inherently racy; your forgot the detail of 'some fresh value',
but since you want to avoid collisions you really want an increment.
But load-store archs cannot do that. Or rather, they need to do:
load Rn, $event
add Rn, Rn, 1
store $event, Rn
But if they're preempted in the middle, two threads will collide and
generate the _same_ increment. Comparing CPU numbers will not fix that.
next prev parent reply other threads:[~2016-04-08 6:41 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-10-27 23:56 [RFC PATCH 0/3] restartable sequences v2: fast user-space percpu critical sections Paul Turner
2015-10-27 23:56 ` [RFC PATCH v2 1/3] restartable sequences: user-space per-cpu " Paul Turner
2015-11-19 16:38 ` Johannes Berg
2015-12-11 12:56 ` Mathieu Desnoyers
2015-10-27 23:57 ` [RFC PATCH v2 2/3] restartable sequences: x86 ABI Paul Turner
2015-10-28 5:03 ` Peter Zijlstra
2015-10-28 5:19 ` Paul Turner
2015-12-11 13:30 ` Mathieu Desnoyers
2015-10-27 23:57 ` [RFC PATCH v2 3/3] restartable sequences: basic self-tests Paul Turner
2016-04-05 20:33 ` Mathieu Desnoyers
2016-04-06 7:43 ` Peter Zijlstra
2016-04-06 13:39 ` Mathieu Desnoyers
2016-04-06 19:25 ` Peter Zijlstra
2015-10-28 14:44 ` [RFC PATCH 0/3] restartable sequences v2: fast user-space percpu critical sections Dave Watson
2015-12-11 12:05 ` Mathieu Desnoyers
2015-12-11 13:39 ` Mathieu Desnoyers
2016-04-06 15:56 ` Andy Lutomirski
2016-04-07 12:02 ` Peter Zijlstra
2016-04-07 14:35 ` Andy Lutomirski
2016-04-07 15:24 ` Peter Zijlstra
2016-04-07 15:39 ` Peter Zijlstra
2016-04-07 15:44 ` Andy Lutomirski
2016-04-07 15:53 ` Peter Zijlstra
2016-04-07 16:43 ` Andy Lutomirski
2016-04-07 20:11 ` Peter Zijlstra
2016-04-07 22:05 ` Andy Lutomirski
2016-04-08 1:11 ` Mathieu Desnoyers
2016-04-08 1:21 ` Andy Lutomirski
2016-04-08 2:05 ` Mathieu Desnoyers
2016-04-08 17:46 ` Mathieu Desnoyers
2016-04-08 21:16 ` Andy Lutomirski
2016-04-08 21:25 ` Linus Torvalds
2016-04-10 14:07 ` Mathieu Desnoyers
2016-04-08 11:02 ` Peter Zijlstra
2016-04-08 15:57 ` Andy Lutomirski
2016-04-08 6:41 ` Peter Zijlstra [this message]
2016-04-08 15:58 ` Andy Lutomirski
2016-04-11 21:55 ` Mathieu Desnoyers
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