From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Victor Kaplansky <VICTORK@il.ibm.com>,
Anton Blanchard <anton@samba.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Frederic Weisbecker <fweisbec@gmail.com>,
LKML <linux-kernel@vger.kernel.org>,
Linux PPC dev <linuxppc-dev@ozlabs.org>,
Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
Michael Ellerman <michael@ellerman.id.au>,
Michael Neuling <mikey@neuling.org>,
Oleg Nesterov <oleg@redhat.com>
Subject: Re: perf events ring buffer memory barrier on powerpc
Date: Mon, 4 Nov 2013 02:00:43 -0800 [thread overview]
Message-ID: <20131104100042.GK3947@linux.vnet.ibm.com> (raw)
In-Reply-To: <20131104090744.GE10651@twins.programming.kicks-ass.net>
On Mon, Nov 04, 2013 at 10:07:44AM +0100, Peter Zijlstra wrote:
> On Sat, Nov 02, 2013 at 08:20:48AM -0700, Paul E. McKenney wrote:
> > On Fri, Nov 01, 2013 at 11:30:17AM +0100, Peter Zijlstra wrote:
> > > Furthermore there's a gazillion parallel userspace programs.
> >
> > Most of which have very unaggressive concurrency designs.
>
> pthread_mutex_t A, B;
>
> char data_A[x];
> int counter_B = 1;
>
> void funA(void)
> {
> pthread_mutex_lock(&A);
> memset(data_A, 0, sizeof(data_A));
> pthread_mutex_unlock(&A);
> }
>
> void funB(void)
> {
> pthread_mutex_lock(&B);
> counter_B++;
> pthread_mutex_unlock(&B);
> }
>
> void funC(void)
> {
> pthread_mutex_lock(&B)
> printf("%d\n", counter_B);
> pthread_mutex_unlock(&B);
> }
>
> Then run: funA, funB, funC concurrently, and end with a funC.
>
> Then explain to userman than his unaggressive program can return:
> 0
> 1
>
> Because the memset() thought it might be a cute idea to overwrite
> counter_B and fix it up 'later'. Which if I understood you right is
> valid in C/C++ :-(
>
> Not that any actual memset implementation exhibiting this trait wouldn't
> be shot on the spot.
Even without such a malicious memcpy() implementation I must still explain
about false sharing when the developer notices that the unaggressive
program isn't running as fast as expected.
> > > > By marking "ptr" as atomic, thus telling the compiler not to mess with it.
> > > > And thus requiring that all accesses to it be decorated, which in the
> > > > case of RCU could be buried in the RCU accessors.
> > >
> > > This seems contradictory; marking it atomic would look like:
> > >
> > > struct foo {
> > > unsigned long value;
> > > __atomic void *ptr;
> > > unsigned long value1;
> > > };
> > >
> > > Clearly we cannot hide this definition in accessors, because then
> > > accesses to value* won't see the annotation.
> >
> > #define __rcu __atomic
>
> Yeah, except we don't use __rcu all that consistently; in fact I don't
> know if I ever added it.
There are more than 300 of them in the kernel. Plus sparse can be
convinced to yell at you if you don't use them. So lack of __rcu could
be fixed without too much trouble.
The C/C++11 need to annotate functions that take arguments or return
values taken from rcu_dereference() is another story. But the compilers
have to get significantly more aggressive or developers have to be doing
unusual things that result in rcu_dereference() returning something whose
value the compiler can predict exactly.
Thanx, Paul
next prev parent reply other threads:[~2013-11-04 10:07 UTC|newest]
Thread overview: 120+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-10-22 23:54 perf events ring buffer memory barrier on powerpc Michael Neuling
2013-10-23 7:39 ` Victor Kaplansky
2013-10-23 14:19 ` Frederic Weisbecker
2013-10-23 14:25 ` Frederic Weisbecker
2013-10-25 17:37 ` Peter Zijlstra
2013-10-25 20:31 ` Michael Neuling
2013-10-27 9:00 ` Victor Kaplansky
2013-10-28 9:22 ` Peter Zijlstra
2013-10-28 10:02 ` Frederic Weisbecker
2013-10-28 12:38 ` Victor Kaplansky
2013-10-28 13:26 ` Peter Zijlstra
2013-10-28 16:34 ` Paul E. McKenney
2013-10-28 20:17 ` Oleg Nesterov
2013-10-28 20:58 ` Victor Kaplansky
2013-10-29 10:21 ` Peter Zijlstra
2013-10-29 10:30 ` Peter Zijlstra
2013-10-29 10:35 ` Peter Zijlstra
2013-10-29 20:15 ` Oleg Nesterov
2013-10-29 19:27 ` Vince Weaver
2013-10-30 10:42 ` Peter Zijlstra
2013-10-30 11:48 ` James Hogan
2013-10-30 12:48 ` Peter Zijlstra
2013-11-06 13:19 ` [tip:perf/core] tools/perf: Add required memory barriers tip-bot for Peter Zijlstra
2013-11-06 13:50 ` Vince Weaver
2013-11-06 14:00 ` Peter Zijlstra
2013-11-06 14:28 ` Peter Zijlstra
2013-11-06 14:55 ` Vince Weaver
2013-11-06 15:10 ` Peter Zijlstra
2013-11-06 15:23 ` Peter Zijlstra
2013-11-06 14:44 ` Peter Zijlstra
2013-11-06 16:07 ` Peter Zijlstra
2013-11-06 17:31 ` Vince Weaver
2013-11-06 18:24 ` Peter Zijlstra
2013-11-07 8:21 ` Ingo Molnar
2013-11-07 14:27 ` Vince Weaver
2013-11-07 15:55 ` Ingo Molnar
2013-11-11 16:24 ` Peter Zijlstra
2013-11-11 21:10 ` Ingo Molnar
2013-10-29 21:23 ` perf events ring buffer memory barrier on powerpc Michael Neuling
2013-10-30 9:27 ` Paul E. McKenney
2013-10-30 11:25 ` Peter Zijlstra
2013-10-30 14:52 ` Victor Kaplansky
2013-10-30 15:39 ` Peter Zijlstra
2013-10-30 17:14 ` Victor Kaplansky
2013-10-30 17:44 ` Peter Zijlstra
2013-10-31 6:16 ` Paul E. McKenney
2013-11-01 13:12 ` Victor Kaplansky
2013-11-02 16:36 ` Paul E. McKenney
2013-11-02 17:26 ` Paul E. McKenney
2013-10-31 6:40 ` Paul E. McKenney
2013-11-01 14:25 ` Victor Kaplansky
2013-11-02 17:28 ` Paul E. McKenney
2013-11-01 14:56 ` Peter Zijlstra
2013-11-02 17:32 ` Paul E. McKenney
2013-11-03 14:40 ` Paul E. McKenney
2013-11-03 15:17 ` [RFC] arch: Introduce new TSO memory barrier smp_tmb() Peter Zijlstra
2013-11-03 18:08 ` Linus Torvalds
2013-11-03 20:01 ` Peter Zijlstra
2013-11-03 22:42 ` Paul E. McKenney
2013-11-03 23:34 ` Linus Torvalds
2013-11-04 10:51 ` Paul E. McKenney
2013-11-04 11:22 ` Peter Zijlstra
2013-11-04 16:27 ` Paul E. McKenney
2013-11-04 16:48 ` Peter Zijlstra
2013-11-04 19:11 ` Peter Zijlstra
2013-11-04 19:18 ` Peter Zijlstra
2013-11-04 20:54 ` Paul E. McKenney
2013-11-04 20:53 ` Paul E. McKenney
2013-11-05 14:05 ` Will Deacon
2013-11-05 14:49 ` Paul E. McKenney
2013-11-05 18:49 ` Peter Zijlstra
2013-11-06 11:00 ` Will Deacon
2013-11-06 12:39 ` Peter Zijlstra
2013-11-06 12:51 ` Geert Uytterhoeven
2013-11-06 13:57 ` Peter Zijlstra
2013-11-06 18:48 ` Paul E. McKenney
2013-11-06 19:42 ` Peter Zijlstra
2013-11-07 11:17 ` Will Deacon
2013-11-07 13:36 ` Peter Zijlstra
2013-11-07 23:50 ` Mathieu Desnoyers
2013-11-04 11:05 ` Will Deacon
2013-11-04 16:34 ` Paul E. McKenney
2013-11-03 20:59 ` Benjamin Herrenschmidt
2013-11-03 22:43 ` Paul E. McKenney
2013-11-03 17:07 ` perf events ring buffer memory barrier on powerpc Will Deacon
2013-11-03 22:47 ` Paul E. McKenney
2013-11-04 9:57 ` Will Deacon
2013-11-04 10:52 ` Paul E. McKenney
2013-11-01 16:11 ` Peter Zijlstra
2013-11-02 17:46 ` Paul E. McKenney
2013-11-01 16:18 ` Peter Zijlstra
2013-11-02 17:49 ` Paul E. McKenney
2013-10-30 13:28 ` Victor Kaplansky
2013-10-30 15:51 ` Peter Zijlstra
2013-10-30 18:29 ` Peter Zijlstra
2013-10-30 19:11 ` Peter Zijlstra
2013-10-31 4:33 ` Paul E. McKenney
2013-10-31 4:32 ` Paul E. McKenney
2013-10-31 9:04 ` Peter Zijlstra
2013-10-31 15:07 ` Paul E. McKenney
2013-10-31 15:19 ` Peter Zijlstra
2013-11-01 9:28 ` Paul E. McKenney
2013-11-01 10:30 ` Peter Zijlstra
2013-11-02 15:20 ` Paul E. McKenney
2013-11-04 9:07 ` Peter Zijlstra
2013-11-04 10:00 ` Paul E. McKenney [this message]
2013-10-31 9:59 ` Victor Kaplansky
2013-10-31 12:28 ` David Laight
2013-10-31 12:55 ` Victor Kaplansky
2013-10-31 15:25 ` Paul E. McKenney
2013-11-01 16:06 ` Victor Kaplansky
2013-11-01 16:25 ` David Laight
2013-11-01 16:30 ` Victor Kaplansky
2013-11-03 20:57 ` Benjamin Herrenschmidt
2013-11-02 15:46 ` Paul E. McKenney
2013-10-28 19:09 ` Oleg Nesterov
2013-10-29 14:06 ` [tip:perf/urgent] perf: Fix perf ring buffer memory ordering tip-bot for Peter Zijlstra
2014-05-08 20:46 perf events ring buffer memory barrier on powerpc Mikulas Patocka
[not found] ` <OF667059AA.7F151BCC-ONC2257CD3.0036CFEB-C2257CD3.003BBF01@il.ibm.com>
2014-05-09 12:20 ` Mikulas Patocka
2014-05-09 13:47 ` Paul E. McKenney
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